1 C sugar
½ t vanilla
¾ C sifted flour
scant ½ t baking powder
scant ¼ t salt
3 T peanut butter
3 T cocoa powder
¼ C chocolate chips
Preheat oven to 180ºC (350ºF). Melt the butter by placing your metal mixing bowl in the preheating oven. While you do that, get your other ingredients ready.
Sift the baking powder and salt into the flour and mix around. Add to the wet ingredients.
Divide the batter into two bowls. Add peanut butter to one half, and cocoa and chips to the other half.
Put the different batters in a checkerboard pattern in an 8" square pan lined with parchment paper. Swirl and spread with a knife or spatula. Bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean.