1. Combine melted butter and brown sugar in a medium bowl; add eggs. Beat at medium speed of an electric mixer until thoroughly combined. Add vanilla; beat well.
2. Combine flour, baking powder, and salt; add to sugar mixture, stirring well.
3. Divide batter in half. Stir cocoa and nuts (if using) into one half; stir peanut butter into other half. (Peanut butter mixture will be thick.)
4. Spoon dollops of each batter alternately into a 9-inch (23cm) square pan coated with cooking spray. Cut through batters in pan with a knife to create a swirled pattern. Bake at 350° F (180°C) for 25 minutes or until a wooden stick inserted in center comes out clean. Remove from oven, and let cool completely on a wire rack. When brownies are completely cooled, cut into squares.
Makes 16 brownies.
Date: November 6, 2003