Preheat oven to 350ºF.
Add liners to two cupcake pans (24 cupcakes total). Place both pans on an angle, using a ramekin or something of similar height underneath one side of the pan to sit on an angle.
Fill half the cupcake cavities with the peanut butter batter and it should sink to the bottom edge of the cupcake liner.
Add the chocolate batter quickly and then remove the ramekin from underneath the cupcake pan.
Use a chopstick or a toothpick to swirl around the peanut butter and chocolate batters until you like the way it looks.
Bake for 15-20 minutes until a toothpick comes out clean when you poke it into a cupcake.
Remove from oven and let cool on a wire rack. Remove from the pan as soon as you can and let cool completely.