Preheat: Set your oven to 350°F and line a 12-cup muffin tin with liners.
Mix Dry Ingredients: In a large bowl, whisk flour, sugar, cocoa, baking soda, and salt.
Combine Wet Ingredients: In another bowl, mix coffee, vinegar, oil, egg, and vanilla.
Combine Mixtures: Pour wet ingredients into dry, mixing just until combined. The batter will be slightly lumpy.
Fill and Bake: Fill liners ⅔ full. Bake 18-22 minutes or until a toothpick comes out clean.
Cool: Let cool in pan for 5 minutes, then transfer to a rack to cool completely.
