Preheat oven to 350°F. Line cupcake tin with liners.
In a large bowl, sift together cocoa powder and flour. Mix in baking powder, baking soda, salt, and sugar.
Add milk, vanilla, eggs, and oil. Mix until just combined.
Add boiling water slowly, mixing on low as you add.
Ladle the batter into the tin. Bake for 18-20 minutes until a toothpick inserted in the center comes out clean.
Let the cupcakes sit in the tins for 10 minutes, then transfer to cooling racks. Allow the cupcakes to cool completely before frosting.
8-inch cake variation (makes two)
Grease and flour two 8-inch cake tins. Bake for 25-30 minutes, until a toothpick inserted in the center comes out clean. Let cool before removing from the tins.