<PAGE>
 
                      [LOGO OF EINSTEIN/NOAH BAGEL CORP.]

                                                                    EXHIBIT 10.3

                                 April 29, 1998


Mr. Robert Hartnett
100 MacFarlane Road
Delray Beach, FL  33483
 
Dear Bob:

     I am pleased to confirm our offer of the position of Chief Executive
Officer of ENBC, reporting to the Board of Directors. Your bi-weekly base salary
will be $11,538.46 ($300,000 annualized) subject to annual performance review.
Additionally, you will participate in an annual bonus plan with a target payout
equal to 50% of base pay ($150,000), which will be paid coincident with other
bonuses following the close of the fiscal year.  One-third of your bonus will be
based on the Company's revenue performance, one-third of your bonus will be
based on Company profitability and one-third of your bonus will be based on
Company overhead, in each case measured against targets to be mutually agreed
upon by you and the Company within two weeks of the date hereof.  No other terms
and conditions of your employment are otherwise affected.

     As an executive officer of the Company, you will continue to be entitled to
participate in the Company's Amended & Restated 1997 Stock Option Plan.  At the
Stock Option Committee meeting on March 26, 1998, you were given a stock option
grant under the 1997 plan of 210,000 shares at an exercise price of $4.5625 per
share.  Such options will vest over a four- year period at the rate of  25% per
year, as further detailed in the Amended & Restated 1997 Stock Option Plan.
Further, these options will be subject to all the provisions of that Plan as it
may be amended from time to time.
 
     Additionally, the Company will reimburse you for all reasonable costs
(closing costs, movement of household goods, househunting and temporary living)
for your relocation to the Denver area. All reimbursed amounts will be tax
protected at 35% to help compensate for the taxes you may incur as a result of
the allowance.  The actual incremental taxes you pay will depend upon the
deductibility of the relocation expenses you incur. You will also be advanced
$30,000 as a miscellaneous relocation allowance payable after your start date.
Should you voluntarily terminate your employment with the Company on or before
one year from your start date in this position, or should your employment be
terminated for cause during such period, you hereby agree to repay all monies
paid you by the Company for relocation, including the $30,000 miscellaneous
relocation allowance.
 
     As a condition of employment, you will be required to enter into the
Company's standard form of  confidentiality and non-compete agreement.
<PAGE>
 
[LOGO OF EINSTEIN/NOAH BAGEL CORP.]

Mr. Robert Hartnett
April 29, 1998
Page 2

     The Company will continue your base salary for one year in the event your
employment is terminated, other than for "cause" (as hereinafter defined),
within four years of the date of this letter.  In addition, in such event you
will be entitled to receive a pro rata portion of your bonus for the year in
which your employment is terminated in such amount as may be reasonably
determined by the Board of Directors based on performance relative to
performance targets for such partial year and the Company will pay for the
continuation of your health insurance coverage for eighteen months (or such
shorter period ending when you are covered under an employer-paid health
insurance plan of a successor employer).  Finally, in such event all Company
stock options held by you at the time of such termination shall also continue to
vest during the one-year period following termination and be exercisable for one
additional year after the end of such one-year period of continued vesting.

     The Company will continue your base salary for two years in the event your
employment is terminated, other than for "cause" (as hereinafter defined), or in
the event of your voluntary termination of employment for "good reason" (as
hereinafter defined), in either case within four years of the date of this
letter but following a "change in control" (as hereinafter defined) of the
Company.  In addition, in such event you will be entitled to receive a pro rata
portion of your bonus for the year in which your employment is terminated in
such amount as may be reasonably determined by the Board of Directors based on
performance relative to performance targets for such partial year and the
Company will pay for the continuation of your health insurance coverage for
eighteen months (or such shorter period ending when you are covered under an
employer-paid health insurance plan of a successor employer). Finally, in such
event all Company stock options held by you at the time of such termination
shall also continue to vest, and be exercisable, during the two-year period
following termination.

     For purposes of this letter agreement, a "change in control" of the Company
shall mean any of the following: (a) the acquisition by any person (including
any syndicate or group deemed to be a "person" under Section 13(d)(3) or
14(d)(2) of the Securities Exchange Act of 1934, as amended (the "Exchange
Act"), or any successor provision, but excluding Boston Chicken, Inc.) of
beneficial ownership, directly or indirectly, through a purchase, merger or
other acquisition transaction or series of transactions, of shares of capital
stock (or other voting securities) of the Company entitling such person to
exercise (i) more than 30% of the total voting power of all shares of capital
stock or other voting securities of the Company entitling the holders thereof to
vote generally in elections of directors ("Voting Power"), with "beneficial
ownership" being determined for this purpose in accordance with Rule 13d-3 under
the Exchange Act as in effect on the date hereof, and (ii) a percentage of
Voting Power in excess of the percentage of Voting Power which Boston Chicken,
Inc. is entitled to exercise by reason of its ownership of capital stock or
other voting securities of the Company, (b) a sale of all or substantially all
of the assets of the Company to any purchaser (the "Purchaser") if any person
other than Boston Chicken, Inc. is the beneficial owner of shares of capital
stock of the Purchaser entitling such person to exercise (i) more than 30% of
the Voting Power of such Purchaser and (ii) a percentage of Voting Power of the
Purchaser in excess of the percentage of Voting Power of the Purchaser which
Boston Chicken, Inc. is entitled to exercise by reason of its ownership of
equity interests
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[LOGO OF EINSTEIN/NOAH BAGEL CORP.]
 
Mr. Robert Hartnett
April 29, 1998
Page 3

or other voting securities in the Purchaser, or (c) the individuals who, as of
the date of any "change in control" of Boston Chicken, Inc., are members of the
board of directors of the Company ceasing for any reason to constitute a
majority of the board of directors of the Company. For purposes of this letter
agreement a "change in control" of Boston Chicken, Inc. shall mean (i) the
acquisition by any person (including any syndicate or group deemed to be a
"person" under Section 13(d)(3) or 14(d)(2) of the Exchange Act, or any
successor provision, but excluding Scott Beck or Saad Nadhir) of beneficial
ownership, directly or indirectly, through a purchase, merger or other
acquisition transaction or series of transactions, of shares of capital stock or
other voting securities of Boston Chicken, Inc. entitling such person to
exercise more than 30% of the Voting Power of Boston Chicken, Inc., (ii) a sale
of all or substantially all of the assets of Boston Chicken, Inc. to any
purchaser (a "BCI Purchaser") if any person other than Scott Beck or Saad Nadhir
is the beneficial owner of shares of capital stock or other voting securities of
the BCI Purchaser entitling such person to exercise more than 30% of the Voting
Power of such Purchaser, or (iii) the individuals who, as of the date of this
letter agreement, are members of the board of directors of Boston Chicken, Inc.
(the "Incumbent Board") cease for any reason to constitute at least a majority
of the board of directors of Boston Chicken, Inc., provided, however, that if
either the election of any new director or the nomination for election of any
new director by Boston Chicken, Inc.'s stockholders was approved by a vote of at
least a majority of the Incumbent Board, such new director shall be considered a
member of the Incumbent Board.

     For purposes of this letter agreement, (a) termination for "cause" shall
mean termination of your employment by reason of any misappropriation of funds
or other property of the Company, commission of any felony or any crime
involving moral turpitude, fraud or theft, or any breach of your confidentiality
or noncompete agreement with the Company, and (b) voluntary termination for
"good reason" shall mean your voluntary termination of employment following any
material reduction of your compensation or benefits or any relocation of your
position to a location more than 60 miles from Golden, Colorado.

     The amounts payable under this letter agreement will be paid without
reduction (except as expressly provided herein with respect to payment of health
insurance premiums), regardless of any compensation paid or payable to you from
any source or which you could have obtained upon seeking other employment,
provided that the Company shall be permitted to make all payments net of any
legally required tax withholding.

     This letter agreement shall be governed by Colorado law.

     You represent and warrant that you are not a party to any non-compete,
confidentiality or similar agreement, whether written or oral, with any person
or entity that would restrict or otherwise affect your employment with the
Company.
<PAGE>
 
[LOGO OF EINSTEIN/NOAH BAGEL CORP.]
 
Mr. Robert Hartnett
April 29, 1998
Page 4

     We are delighted that you have decided to move to Colorado and assume this
important position.  We look forward to what we believe will be an enormous
contribution by you to ENBC and we will do all that we can to ensure your
success.

     Again, congratulations!

                                               Sincerely,



                                               /s/ Scott A. Beck
                                               ------------------------------
                                                   Scott A. Beck
                                                   Chairman


I hereby accept the offer as stated above:

Signature: /s/ Robert M. Hartnett              Date: 4/29/98
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