Preheat oven to 350 degrees F. Line an 8x8 baking pan with parchment paper or spray with cooking spray. Set aside.
Combine 1 cup flour and baking powder in a small bowl. Set aside.
In another small bowl, combine the ¼ cup flour and cocoa powder. Set aside.
Pour melted butter into a medium bowl. Add sugar and water and whisk to combine. Whisk in egg, oil, and vanilla. Stir in the combined flour and baking powder.
In a small bowl, mix peanut butter with ½ cup of the batter. Set aside. Stir the flour and cocoa powder mixture into the plain batter; fold in chocolate chips. Pour the chocolate batter into prepared pan.
Drop peanut butter batter in tablespoonfuls over the chocolate batter in the pan. Use a toothpick or butter knife to swirl peanut butter batter into the chocolate batter.
Bake for 20 to 25 minutes or until top springs back when lightly touched and a toothpick inserted near the center comes out clean. Place the pan on a wire rack to cool.
Recipe by Baked In at https://www.baked-in.com/2012/06/11/peanut-butter-swirl-brownies/