Preheat oven to 350 degrees Fahrenheit. Line muffin tins with paper cupcake liners. (About 18.)
Sift together the sugar, flour, cocoa, baking powder, baking soda, and salt into a large bowl. (Sifting will create a lighter/softer cupcake. If you don't have a sieve, whisk these ingredients well.)
Add the egg, milk, oil, and vanilla. Stir until incorporated.
Heat ½ cup of water in a large glass measuring cup in the microwave until boiling (1-2 minutes). Pour boiling water into batter and mix until fully incorporated. The texture of the batter should be consistent throughout. Batter will be thin, but shouldn't have watery spots.
Use a #24 scoop to fill prepared cupcake liners ⅔ full (just barely over halfway full). Do NOT overfill. Bake for 20-22 minutes. To test doneness -- a toothpick will come clean out of the center or the cake will spring back when touched.
Cool completely and decorate with desired frosting. I used this Chocolate Cream Cheese Frosting, but this Chocolate Buttercream Frosting would be great too.
