01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners.
02 - Sift together flour, cocoa powder, sugar, baking powder, baking soda, and salt in a large bowl.
03 - Whisk eggs, milk, oil, and vanilla extract in a separate bowl until smooth.
04 - Add wet ingredients to dry and mix until just incorporated.
05 - Gradually stir in hot water until batter is smooth but thin.
06 - Divide batter evenly into liners, filling each two-thirds full.
07 - Bake for 16 to 18 minutes or until a toothpick inserted in the center comes out clean.
08 - Let cupcakes cool in tin for 5 minutes, then transfer to a wire rack to cool completely.
09 - Beat butter until creamy. Add powdered sugar, cocoa powder, milk, vanilla, and salt; beat until fluffy.
10 - Once cooled, frost cupcakes as desired.