Question: Why is an extra egg and additional square of chocolate added to the Boston Cooking School brownie recipe?
About the article "Chocolate brownie", section "History".

Candidate passages:
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. 
A hedgehog slice is an uncooked flat, square or bar-shaped chocolate snack/ dessert, similar to a fudgey chocolate brownie but with alternating lighter and darker areas.
The darker areas are chocolate flavoured.
The lighter areas are bread crumbs, crushed biscuit, rice puffs, or similar.
Nuts may also be added.
It usually has a topping of chocolate icing on which may be sprinkled coconut, Hundreds and Thousands, or other kinds of sprinkles (e.g.
chocolate or coffee flavoured, etc.
It is widely available in cafes and bakeries in Australia. 
Possibly one of Hershey's best-known products due to long-running massive advertising campaigns.
Peanut Butter and Milk Chocolate Peanut Butter and White Chocolate Crispy Crunches Reese's Select Clusters Peanut Butter and Milk Chocolate Big Cup, a larger Peanut Butter Cup with milk chocolate Peanut Butter and White Chocolate Big Cup, a larger Peanut Butter Cup with white chocolate Brownie, a brownie containing peanut butter and chocolate Egg, Peanut Butter Cup in the shape of an egg Heart, Peanut Butter Cup in the shape of a heart Miniatures, A variation of the classic Reese's Peanut Butter Cup Pumpkin, Peanut Butter Cup in the shape of a pumpkin Tree, Peanut Butter Cup in the shape of a tree Dark Chocolate, A Reese's Peanut Butter using Hershey's Dark Chocolate Whitey Tightey Chocolate  
"This recipe is also known as "Death by Chocolate, as it contains a considerible amount of chocolate, if topped with chocolate ice cream and lashings of whipped cream."
In addition to Death By Chocolate, the many variations include pudding fudge cake, made with chocolate cake mix, chocolate pudding, and chocolate chips.
Alternatively, "fudge cake" is a term in the American South to refer to a dense, single-layer chocolate cake served with or without icing.
It is similar to a brownie, although moister with more chocolate.