Photo courtesy of Derf, from Food.com, since mine didn't turn out nearly as thick as theirs!
Here's the recipe!
1/2 cup butter or 1/2 cup margarine, softened
1 cup sugar
1 teaspoon vanilla extract
1 1/2 cups mashed ripe bananas (about 3 medium)
1 1/2 cups all-purpose flour
1 teaspoon baking powder
1 teaspoon baking soda
1/2 teaspoon salt
1/4 cup baking cocoa
I also topped with a hand full of chocolate chips to make it look a little prettier (ok fine, I did change the recipe just a bit, but I don't think this counts!)
In a mixing bowl, cream butter and sugar. Add egg and vanilla; beat until thoroughly combined. Blend in the bananas. Combine the flour, baking powder, baking soda and salt; add to creamed mixture and mix well. Divide batter in half. Add cocoa to half; spread into a greased 13-in x 9-in pan. (I think this is too large a pan, mine turned out really thin) Spoon remaining batter on top and swirl with a knife. (This also didn't turn out well, it didn't look like when you swirl a brownie batter with cream cheese, etc.) Bake at 350 degrees for 25 minutes or until the bars test done. Cool and enjoy!
Let me know what you think!