Preheat your oven to 350°F (175°C) and line a muffin pan with cupcake liners.
In a bowl, whisk together the flour, cocoa powder, baking soda, and sugar until well combined.
In another bowl, mix eggs, milk, vegetable oil, and vanilla extract until smooth.
Combine the wet ingredients into the dry ingredients and stir gently. Fold in chopped Reese’s cups.
Fill each cupcake liner two-thirds full with batter and bake for 18–20 minutes or until a toothpick comes out clean.
Let cupcakes cool completely before preparing the frosting by beating together butter and peanut butter until fluffy. Gradually add powdered sugar and heavy cream until smooth.
Frost cooled cupcakes generously and decorate as desired.
