01 - Preheat the oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - In a large bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt until evenly mixed.
03 - In a separate bowl, whisk eggs, whole milk, vegetable oil, and vanilla extract until fully incorporated.
04 - Gently fold the wet mixture into the dry ingredients until just combined, avoiding overmixing.
05 - Stir in hot water or coffee until the batter is smooth and thin.
06 - Distribute the batter evenly among the liners, filling each about 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 cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.
09 - Beat softened butter until creamy. Gradually add powdered sugar, cocoa powder, vanilla extract, and salt. Mix well, then add milk gradually until desired consistency is reached.
10 - Apply frosting to cooled cupcakes as desired.