brownies [brou-neez] noun
A chocolate bar cookie.
Fannie Merrit Farmer published a recipe in 1896 called a brownie and it was indeed brown, being flavored with molasses, but it did not contain any cocoa or chocolate. In her revised 1905 Boston Cooking-School Cook Book she presented a chocolate version. There is no clear-cut origin story. What they have in common is that they are usually prepared in an 8-inch or 9-inch square pan; larger recipes can be made in 13 x 9-inch pans or half-sheet pans.
Brownies typically do not contain chemical leaveners, or of they do it is a very small amount. They are a classic American dessert that can range in texture from dense and truffle-like to light and cakey, with nuts and without. When they include nuts, they are often walnuts. There is no right or wrong – there is room for brownies of every stripe!
We may never trust the Merriam-Webster Dictionary on-line version again. We say this in jest, however, look at what they say, and in which order they say it:
Definition of BROWNIE
1: a legendary good-natured elf that performs helpful services at night
2 capitalized: a member of a program of the Girl Scouts for girls in the first through third grades in school
3: a small square or rectangle of rich usually chocolate cake often containing nuts