Heat oven to 350F. Beat butter, sugar,
vanilla and egg in a large bowl with electric mixer on medium speed, or mix with
spoon.  Stir in flour and salt.  Shape dough into 1” balls.  Place about 2” apart
on ungreased cookie sheet. 

Press thumb or end of a wooden spoon into center
of each cookie ball.  Do not press all the way to the cookie sheet.  Bake 7-11
minutes, or till edges are firm.  Quickly remake indentations with end of a
wooden spoon if necessary.  Immediately remove from cookie sheet to wire rack. 
Cool completely, about 30 minutes. 

Meanwhile, make Chocolate
Ganache Filling – Heat 1/4 cup heavy cream in a
1-quart (or smaller) saucepan over medium heat, stirring, until steaming. 
Remove from heat; stir in 1 cup semi-sweet chocolate chips till
melted.  Cool about 10 minutes, or till thickened.  Spoon 3/4 of ganache into a
plastic resealable sandwich bag.  Spoon remaining ganache into another bag. 
Seal bags, squeezing out air.  Snip one corner of each bag: the smaller bag
with a very tiny snip for drizzling and the larger bag a larger snip to squeeze
out ganache into cookie cavities.  Fill cavities with ganache.  Sprinkle each
cookie with some of the Heath bar bits.  Drizzle remaining ganache from smaller
bag onto cookie tops.  Yield: about 3-1/2 dozen