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. x 2-in. baking pan. Spoon remaining batter on top and swirl with a knife. Bake at 350 degrees for 25 minutes or until the bars test done.