3. Spoon 2 tablespoons chocolate batter into each lined cup, followed by 1 tablespoon peanut butter filling. Repeat with another tablespoon of batter, and top with 1 teaspoon filling. Swirl top of cupcake batter and filling with a wooden skewer or toothpick.
4. Bake, rotating tin halfway through, until a cake tester inserted in centers comes out with only a few moist crumbs attached, about 40 minutes. Transfer tin to a wire rack to cool completely before removing cupcakes. Cupcakes can be stored up to 3 days at room temperature in airtight containers.