01 - Preheat oven to 350°F. Line a 12-cup muffin tin with cupcake liners.
02 - Whisk flour, cocoa powder, sugar, baking powder, baking soda, and salt in a large bowl until well blended.
03 - In a separate bowl, whisk eggs, whole milk, vegetable oil, and vanilla extract until thoroughly combined.
04 - Pour wet mixture into dry ingredients and mix gently until just incorporated.
05 - Stir in hot water until batter is smooth and slightly thin. Avoid overmixing.
06 - Distribute batter evenly among cupcake liners, filling each about 2/3 full.
07 - Bake 16 to 18 minutes, or until a toothpick inserted into the center emerges clean.
08 - Allow cupcakes to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.
09 - Cream butter until light. Sift in powdered sugar and cocoa powder. Add milk, vanilla, and salt. Beat until fluffy and smooth. Frost cooled cupcakes as preferred.