Preheat the oven to 350˚F, positioning the rack in the center. Line a standard 12-cup muffin pan with paper liners. Combine the flour, sugar, cocoa powder, baking soda, and salt in a large bowl, whisking thoroughly to break up any cocoa lumps. In a separate bowl, blend the warm coffee (or buttermilk), vinegar, oil, egg, and vanilla extract until fully incorporated. Pour the wet mixture into the bowl of dry ingredients, whisking gently until just combined and no streaks of flour remain; the batter will appear slightly runny and lumpy. Divide the batter evenly among the cupcake liners, filling each about two-thirds full—avoid overfilling. Bake for 18–22 minutes (20 minutes is ideal) or until a toothpick inserted into the center emerges clean. Let the cupcakes cool in the pan for 5 minutes before transferring them to a wire rack to cool completely before applying frosting.