Black Bottom Banana Bars

1/2 cup butter softened
1 cup sugar
1 egg
1 tsp vanilla
1 1/2 cups (3) mashed bananas
1 1/2 cups flour
1 tsp baking powder
1 tsp baking soda
1/2 tsp salt
1/4 cup unsweetened cocoa

cream butter and sugar, add egg and vanilla beat till combined. blend in bananas. combine dry ingredients and add to creamed mixture, mix well. Divide mixture in half and add cocoa to one half.spread cocoa batter in bottom of a greased 13×9 inch pan. spoon remaining batter on top and swirl with a knife. bake at 350 for 25 minutes