For the muffins:
6 tbsp. unsalted butter, at room temperature
2/3 cup semisweet chocolate chips, divided
2 cups all-purpose flour
2/3 cup sugar
1/3 cup unsweetened cocoa powder
1 tbsp. baking powder
½ tsp. baking soda
½ tsp. salt
1¼ cups buttermilk
1 large egg
2 tsp. vanilla extract
For the peanut butter swirl:
1/3 cup creamy peanut butter
1 tbsp. unsalted butter, melted
½ cup confectioners’ sugar
1 tbsp. milk
Preheat the oven to 350˚ F. Line muffin tins with paper liners. To make the chocolate muffin batter, combine the butter and 1/3 cup of the chocolate chips in a heatproof bowl set over a pot of simmering water. Heat, stirring occasionally, until the chocolate and butter are melted and smooth. (Alternatively you can microwave in 20-30 second intervals, stirring in between, until melted and smooth.) Set aside. In the bowl of an electric mixer combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Whisk or mix briefly to combine. In a liquid measuring cup, combine the buttermilk, egg and vanilla and whisk until combined. With the mixer on low speed, blend in the buttermilk mixture just until combined. Blend in the melted chocolate mixture. Stir in the remaining chocolate chips with a spatula.
Divide the batter between the prepared paper liners, filling each no more than 2/3 full. (These muffins will rise a lot!) To make the peanut butter swirl, combine the peanut butter and butter in a heatproof bowl and microwave briefly until smooth. Whisk in the confectioners’ sugar and milk until smooth. Drop a small spoonful of the peanut butter mixture on top of the chocolate muffin batter. Swirl together using a skewer or knife.
Bake 20-22 minutes or until a toothpick inserted in the center comes out clean. Let cool in the pan 10 minutes, then transfer to a wire rack to cool completely.