Question: Why does the Boston Cooking School brownie recipe call for an extra egg and square of chocolate?
About the article "Chocolate brownie", section "History".

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A chef at Chicago's Palmer House Hotel created the confection after Bertha Palmer requested a dessert for ladies attending the Chicago 1893 World Fair; it should be, she said, smaller than a piece of cake, though still retaining cake-like characteristics and easily eaten from boxed lunches.
These first brownies featured an apricot glaze and walnuts, and they are still being made at the hotel according to the original recipe.
The earliest published recipes for a brownie like those of today appeared in the "Home Cookery" (1904, Laconia, NH), "Service Club Cook Book" (1904, Chicago, IL), "The Boston Globe" (April 2, 1905 p. 34) and the 1906 edition of "The Boston Cooking School Cook Book" by Fannie Merritt Farmer.
These recipes produced a relatively mild and cake-like brownie.
The name "brownie" first appeared in the 1896 version of the cookbook, but this was in reference to molasses cakes baked individually in tin molds, not true brownies.
A second recipe appeared in 1907 in "Lowney’s Cook Book", by Maria Willet Howard and published by the Walter M. Lowney Company of Boston, Massachusetts.
This recipe added an extra egg and an additional square of chocolate to the Boston Cooking School recipe, creating a richer, fudgier brownie.
The recipe was named Bangor Brownies, possibly because it was created by a woman in Bangor, Maine.
The Bangor Brownie went on to be rated third in the top 10 snacks a few years later.
German chocolate cake, originally German's chocolate cake, is a layered, chocolate cake from the United States filled and topped with a coconut-pecan frosting.
It owes its name to an American chocolate maker named Sam German, who developed a formulation of dark baking chocolate that came to be used in the cake recipe.
Sweet baking chocolate is traditionally used for the chocolate flavor in the actual cake, but few recipes call for it today.
The filling and/or topping is a caramel made with egg yolks and evaporated milk; once the caramel is cooked, coconut and pecans are stirred in.
Occasionally, a chocolate frosting is spread on the sides of the cake and piped around the circumference of the layers to hold in the filling.
Maraschino cherries are occasionally added as a garnish. 
The earliest documented recipe for chocolate brownies referred to them as "Bangor Brownies".
Fanny Farmer invented "brownies" in her 1896 cookbook, but these were molasses-flavored, had a nut on top, and were baked in individual pans.
The first recipe for what we'd recognize today as chocolate brownies was published in the "Boston Daily Globe" on 2 April 1905, pg.
34 and read:"BANGOR BROWNIES.
Cream 1/2 cup butter, add 2 eggs, 1 cup sugar, 2 squares of chocolate (melted), 1/2 cup broken walnuts meats, 1/2 cup flour.
Spread thin in buttered pans.
Bake in moderate oven, and cut before cold.
"The 1907 "Lowney's Cook Book", published by the Walter Lowney Chocolate Co., contained two chocolate brownie recipes.
The one with extra chocolate, and baked in a pan, it also called "Bangor Brownies".
The use of the term in printed recipes continued into the 1950s.
The "Appledore Cookbook" of 1872 included a recipe for "Bangor Cake", repeated in the "Woman's Suffragette Cookbook" of 1886, and others as late as 1916.
Two varieties of plum, the "Mclaughlin" and the "Penobscot", were first identified in the garden of John Mclaughlin of Bangor in 1846, and publicized the same year in A. J.
Downing's "The Horticulturalist".
The Mclaughlin had become the most prominent American-cultivated plum by the 1850s, surpassing all others in its "rich and luscious flavor" according to the "Magazine of Horticulture".
Both continue to be grown throughout North America and Europe. 
Possibly one of Hershey's best-known products due to long-running massive advertising campaigns.
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In March 2005, Guylian established a Guinness World Record by building the world's largest chocolate sculpture of an Easter egg on the market square in Sint-Niklaas (Belgium).
26 Guylian master chocolatiers worked continuously for eight days to create this giant chocolate egg.
The sculpture measured 8.32 meters high and 6.39 meters wide and was constructed using 1,950 kilos of chocolate or about 50,000 chocolate Sea Shell Bars.