FOR CUPCAKES

1. Preheat the oven. Set the oven to 350 degrees F. Line a muffin pan with paper liners.
2. Cream the butter. In the bowl of a stand mixer fitted with the paddle attachment, or large mixing bowl with electric beaters, cream together the butter and sugar on medium speed for about 3 minutes until fluffy, stopping partway through to scrape down the sides of the bowl with a spatula.
3. Mix the wet ingredients. In a glass measuring cup with a spout, measure the buttermilk. Add the vanilla, stir and set aside.
4. Mix the dry ingredients. In another large bowl, combine the flour, cocoa powder, baking soda and salt. Using a spatula, stir about 10 times to combine well. Set aside.
5. Add the egg. Crack the egg into a small bowl. Add the egg to the butter mixture. Mix on medium speed for 1 minute.
6. Add the dry ingredients to the wet ingredients. Add half the cocoa mixture to the butter mixture. Mix on medium-low speed for 30 seconds until combined. Add half the buttermilk mixture and mix. Repeat until all the ingredients are combined.
7. Fill the pan. Using an ice cream scoop, divide the batter evenly among the prepared muffin pan, filling each about three-quarters full.
8. Bake. Carefully place the pan onto the center rack of the preheated oven. Bake for 18-20 minutes, rotating the pan halfway through to ensure even cooking. They are ready when you can press your finger into the top of a cupcake and it springs back.

FOR FROSTING

Combine butter and peanut butter in a mixer, beating on high for 1 minute. Add powdered sugar, slowly, beating on low. Add milk as needed, one tablespoon at a time. Cool in refrigerator until the cupcakes have completely cooled, about 1 hour. (Store any leftover frosting in the fridge for up to 2 weeks.)