01 - Set oven to 350°F and line a 12-cup muffin tin with paper liners.
02 - In a large bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.
03 - In a separate bowl, whisk eggs, milk, oil, and vanilla extract until fully blended.
04 - Gently fold the wet ingredients into the dry mixture until just combined without overmixing.
05 - Stir in hot water or coffee until the batter is smooth and thin in consistency.
06 - Divide batter evenly among liners, filling each approximately two-thirds full.
07 - Bake for 16 to 18 minutes or until a toothpick inserted in the center comes out clean.
08 - Allow cupcakes to rest in the pan for 5 minutes before transferring to a wire rack to cool thoroughly.
09 - Whip butter until creamy; gradually add powdered sugar, cocoa powder, vanilla, and salt, mixing well. Slowly incorporate milk to reach desired texture.
10 - Once cooled, apply frosting evenly atop cupcakes as desired.