Preheat the Oven: Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners.
Mix Dry Ingredients: In a large bowl, whisk together all-purpose flour, cocoa powder, baking powder, baking soda, and salt.
Combine Wet Ingredients: In another bowl, beat together the egg, buttermilk, vegetable oil, and vanilla extract until smooth.
Add Dry to Wet: Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Be careful not to overmix.
Incorporate Hot Water or Coffee: Slowly add the hot water or coffee to the batter. It may look thin, but this creates a rich, moist texture.
Fill Cupcake Liners: Pour the batter evenly into the cupcake liners, filling each about 2/3 full.
Bake: Bake for 18-20 minutes or until a toothpick inserted in the center comes out clean.
Cool: Let the cupcakes cool in the pan for a few minutes, then transfer to a wire rack to cool completely.