1. Preheat your oven to 350°F (175°C) and line a 12-cup muffin pan with cupcake liners.
2. In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt until well combined.
3. Add the buttermilk, vegetable oil, egg, and vanilla extract to the dry ingredients. Beat on medium speed until smooth and well blended.
4. Carefully stir in the boiling water until the batter is smooth and slightly runny. This will help the cupcakes stay moist.
5. Pour the batter evenly into the prepared cupcake liners, filling each about two-thirds full.
6. Bake for 18-20 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Allow them to cool completely on a wire rack.
7. In a separate bowl, beat the softened butter and peanut butter together until creamy and smooth.
8. Gradually add the powdered sugar, milk, and vanilla extract, beating until the frosting is fluffy and spreadable.
9. Once the cupcakes are cool, frost them generously with the peanut butter frosting. Feel free to get creative with your piping techniques!

Pro tip from my kitchen to yours: If the frosting is too thick, add a little more milk, a tablespoon at a time, until you reach your desired consistency.