1In a mixing bowl cream butter and sugar. Add egg and vanilla then beat until smooth and fluffy. Add the bananas and beat until they are mashed. Combine flour, baking powder, baking soda and salt then add to creamed mixture. Divide batter in half then add cocoa to half of the batter. Spread cocoa part into a greased 9 x 13 in. pan. Spoon remaining batter on top and at this point you can put nuts or chocolate chips on top.
Bake at 350°F for 25 minutes or until toothpick inserted near center comes out clean. After it has cooled you can cut into bars.