Step 1

Preheat oven to 350 degrees Fahrenheit. Line a 9x9x2-inch baking pan with foil, extending foil up over the edges of the pan. Lightly coat foil with nonstick spray. Set aside.

Step 2

In a medium saucepan, melt butter over low heat; remove from heat. Whisk in sugar and the water. Whisk in egg, oil and vanilla until combined. Stir in 1 cup of the flour and the baking powder until combined (batter will be thin at this point.) Place peanut butter in a small bowl; gradually whisk in 1/2 cup of the batter until smooth. Set aside. In another bowl, combine the remaining 1/4 cup flour and the cocoa powder. Stir into the plain batter; stir in chocolate pieces. Pour chocolate batter into prepared pan.

Step 3

Drop peanut butter batter in small mounds over chocolate batter in pan. Using a thin metal spatula, swirl batters together. Bake for 20 to 25 minutes or until top springs back when lightly touched and a toothpick inserted near the center comes out clean. Cool completely in pan on a wire rack. Cut into bars.