In a mixing bowl, cream butter and sugar. Add egg and vanilla; beat until thoroughly combine. Blend in 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 inch x 9 inch x 2 inch baking pan. Spoon remaining batter on top and swirl with a knife. Bake at 350 °F for 25 minutes or until the bars test as done. Let them cool.