Quick prep - Preheat oven to 350˚F with the rack in the center of the oven. Fill a 12-count muffin tin with cupcake liners.
Mix dry ingredients - In a large mixing bowl, thoroughly whisk together dry ingredients: flour, sugar, cocoa powder, baking soda, and salt. Break up any larger cocoa clumps with your whisk.
Mix wet ingredients - In a second mixing bowl, add the wet ingredients: coffee, vinegar, oil, egg, and vanilla, and whisk until well blended.
Combine batter - Pour wet ingredients over dry ingredients and whisk just until the ingredients come together and no dry streaks remain. The batter will be a little loose and lumpy.
Fill and bake - Use an ice cream scoop to portion batter, filling each cup ⅔ full (do not over-fill). Bake 18-22 min (20 minutes was perfect), until a toothpick inserted in the center comes out clean. Don't overbake. Cool 5 minutes in the pan, then transfer to a rack to cool completely to room temperature before frosting.
