Coat a 9-inch square baking pan with cooking spray; dust with 1 tablespoon flour. Set aside.
Cream margarine. Gradually add sugar; beat at medium speed of a mixer until well-blended. Add vanilla, egg whites, and egg; mix until blended. Combine 1 1/2 cups flour and baking powder; with mixer at low speed, add dry ingredients to creamed mixture. Reserve 1 1/2 cups of batter; pour remaining batter into a bowl. Stir cocoa into reserved batter; stir peanut butter into remaining batter. Spoon cocoa batter alternately with peanut butter batter into prepared pan. Swirl together using the tip of a knife. Bake at 350° for 30 minutes or until a wooden pick inserted in center comes out clean.
This makes a very dense cake. The batter looked like normal cake batter until I split it and added the cocoa powder and the peanut butter. The batter with the cocoa became very dry, and the batter with the peanut butter became very thick. When I poured it into the cake pan (I did a 9 inch round), it looked more like cookie dough than cake batter. Consequently, I couldn't do the knife swirl very well, so it was more like patches of chocolate within the peanut butter. It was a good cake, but I wouldn't make it for friends.
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