For the Muffins:
6 tablespoons Unsalted Butter, at room temperature
2/3 cup Semisweet Chocolate Chips, divided
2 cups Flour
2/3 cup Sugar
1/3 cup Unsweetened Cocoa Powder
1 tablespoon Baking Powder
1/2 teaspoon Baking Soda
1/2 teaspoon Salt
1 1/4 cups Buttermilk
2 teaspoons Vanilla Extract
Combine the butter and 1/3 cup of the chocolate chips in a heatproof bowl. Heat them over a double-boiler or in a microwave until melted and smooth, then 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 to incorporate. With the mixer on low speed, blend in the buttermilk mixture until just combined. Blend in the melted chocolate mixture. Stir in the remaining chocolate chips with a spatula. Divide the batter between the individual paper-lined cups of a muffin/cupcake tin, filling each cup no more than 2/3 full.
For the Peanut Butter Swirl:
1/3 cup Creamy Peanut Butter
1 tablespoon Unsalted Butter, melted
1/2 cup Powdered Sugar
1 tablespoon Milk
In a heat-proof bowl, combine the peanut butter and butter. Microwave briefly until smooth. Whisk in the powdered sugar and milk until smooth. Drop a small spoonful of the peanut butter mixture on top of each muffin. Swirl together using a skewer or knife.
Bake the muffins at 350 degrees for 20-22 minutes, or until a toothpick inserted in the center comes out clean. Let them cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.