Prep oven and pans. Position a rack in the center of your oven. Pre-heat oven to 350°F. Prepare two muffin pans with cupcake liners.
Combine dry ingredients. In a large mixing bowl, sift together the sugar, flour, cocoa, baking powder, and baking soda. Stir in the salt. Set aside.
Combine wet ingredients. In a second large mixing bowl, whisk together the eggs, milk, sour cream, oil, vanilla extract until well combined.
Combine wet and dry ingredients. Add the wet ingredients to the flour mixture, and stir with a rubber spatula, together just until the last of flour has been incorporated.
Add boiling water. Stir in boiling water to batter and stir to combine. (batter will be thin).
Bake cupcakes. Fill each cupcake well about 3/4 cup full of batter. Bake for 18-22 minutes or until wooden pick inserted in center comes out clean, rotating pans halfway through baking. Remove cupcakes from the oven and cool in the pans for 5 minutes, then transfer to a cooling rack and cool on a wire rack. Cool completely. Repeat with the additional cupcake batter if needed. (about 6 additional cupcakes)