Black-Bottom Banana Bars

1/2 cup butter or margarine (softened)
1 cup sugar
1 egg
1 tsp. vanilla
1 1/2 cups mashed ripe bananas (about 3 bananas)
1 1/2 cups all purpose flour
1 tsp. baking powder
1 tsp. baking soda
1/2 tsp. salt
1/4 cup baking cocoa

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 for 25 minutes or until the bars test done. Cool. Yield: 2 1/2 to 3 dozen.