Preheat oven to 350˚F. Line muffin tins with parchment cups.
In a medium bowl, mix together the sugar, flour, cocoa, baking powder, baking soda, salt and espresso powder.
Whisk egg and add milk and vanilla. Melt butter and add to egg mixture.
Pour liquid ingredients into dry ingredients. Mix together. Add boiling water to mixture and blend with a whisk until well combined.
Pour into prepared parchment lined cups about ¾ full. The batter will be thin, so you may want to use a liquid measuring cup to help pour.
Bake for 20-22 minutes. Cool on a wire rack in muffin tins, then remove when cool to touch.