In a large bowl, cream together softened butter, egg, sugar and vanilla until light and fluffy. In a separate bowl, combine all the dry ingredients: flour, baking powder, salt and cocoa powder.

Using a fine mesh strainer, sift in dry ingredients into the batter in small batches, alternating with buttermilk. Whisk batter after each addition. Then, gently fold in Reese's candies.

Fill liners two-thirds full. Bake in the preheated oven until tops are set, about 23-25 minutes. Remove the cupcakes from the pan and onto a cooling rack to cool completely.

For the butter cream: whisk together the butter and peanut butter until light and fluffy; scrape down sides of bowl often to get everything incorporated! Add a few tablespoons of cream to thin out mixture, if needed, then gradually whisk in the confectioner's sugar. Whisk for several minutes until light and fluffy.

Frost cooled cupcakes with butter cream and garnish with additional candies. Keep cupcakes stored in airtight container or covered well with cling wrap so that they don't dry out!