Preheat oven to 350. Line a 9x9x2-inch baking pan with foil, extending foil up over the edges of the pan. Lightly coat foil with nonstick cooking spray. Set aside.
In a medium saucepan, melt butter over low heat; remove from heat. Whisk in sugar and 1/3 cup cold 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 small bowl, combine the remaining 1/4 cup flour and the cocoa powder; stir into the plain batter. Stir chocolate pieces into chocolate batter; pour into prepared pan.
Drop peanut butter batter in small mounds over chocolate batter in pan. Using a thin metal spatula, swirl the batters together. Bake 20-25 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 20 bars.
Notes: If using Sugar Substitutes - Follow package directions to use product amount equivalent to 3/4 cup sugar. Decrease baking time to 15-18 minutes.
Flour Substitutes: 3/4 cup whole grain oat flour, whole grain corn flour, or white bean flour for 3/4 cup of the all purpose flour. Or substitute 1/2 cup full fat almond meal/flour for 1/2 cup of the all purpose flour. Or substitute 1 1/4 cups high fiber flour or gluten-free baking flour (if using gluten-free baking flour, increase water to 1/2 cup) for all of the all purpose flour. If using two flours, stir them together before adding to recipe.