Combine cream butter and sugar in a mixing bowl.
Add egg and vanilla; beat until thoroughly combined.
Blend in the bananas. Add 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 9x13-inch baking pan.
Spoon remaining batter on top and swirl with a knife.
Bake at 350 for 25 minutes or until the bars test done. Let stand and cool.
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