01 - Preheat oven to 350°F. Line a 12-cup muffin tin with cupcake liners.
02 - In a large bowl, whisk together flour, cocoa powder, granulated sugar, brown sugar, baking powder, baking soda, and salt.
03 - In a separate bowl, whisk eggs, milk, vegetable oil, and vanilla extract until smooth.
04 - Add the wet mixture to the dry ingredients and stir until just combined.
05 - Gently mix in hot water or coffee until the batter is smooth and thin.
06 - Divide batter evenly among liners, filling each about two-thirds full.
07 - Bake for 16 to 18 minutes, or until a toothpick inserted into the center comes out clean.
08 - Allow cupcakes to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
09 - Beat butter until creamy. Gradually add powdered sugar, cocoa powder, milk, vanilla extract, and salt. Beat until light and fluffy, adjusting milk for desired consistency.
10 - Once fully cooled, frost cupcakes generously using a piping bag or spatula.