Preheat the oven to 350°F (177°C). Line a 12-count muffin pan with cupcake liners. This recipe yields about 14 to 16 cupcakes, so line a second muffin pan with a few more liners or bake in batches.

Whisk the flour, cocoa powder, baking soda, baking powder, and salt together in a medium bowl. Set aside. In a large bowl, preferably with a pour spout, whisk the oil, sugar, egg, vanilla, and buttermilk together until combined. Pour the dry ingredients into the wet ingredients, add the hot coffee/water, and whisk until the batter is completely combined. Batter is thin.

Pour the batter into the liners, filling only 2/3 full to avoid spilling over the sides.

Bake for 20–22 minutes, or until a toothpick inserted in the center comes out clean.

Allow cupcakes to cool in the pan for 10 minutes, and then transfer to a cooling rack to cool completely. Cupcakes must be completely cooled before decorating.

With a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter on medium speed until creamy, about 2 minutes. Add the peanut butter, confectioners’ sugar, heavy cream, vanilla extract, and salt. Beat on low speed until combined, then increase to high speed and beat for 3 full minutes. Add up to 1/4 cup more confectioners’ sugar if frosting is too thin or another Tablespoon of cream if frosting is too thick.

Frost cooled cupcakes. I used a Wilton 1M piping tip. Top with crushed Reese’s Pieces, if desired.

Store leftovers, covered, in the refrigerator for up to 3 days. I recommend a cupcake carrier for storing decorated cupcakes.