Preheat the oven to 350°F (177°C). Line a 12-cup 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, 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 ⅔ full to avoid spilling over the sides. You should have enough batter for 14 to 16 cupcakes.

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 wire rack to cool completely. Cupcakes must be completely cooled before decorating.

With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. Add the confectioners’ sugar, heavy cream/milk, and vanilla extract. Beat on low speed for 30 seconds, then increase to medium-high speed and beat for 2 full minutes.

Taste. Add a pinch of salt if frosting is too sweet. Add up to ½ cup more confectioners’ sugar if frosting is too thin, or more cream/milk—a tablespoon at a time—if frosting is too thick.

Frost cooled cupcakes and top with sprinkles, if desired. You can swipe the frosting on with an icing knife or use a piping tip.

Store leftover cupcakes covered at room temperature for 1 day or in the refrigerator for 3 days.