Heat oven to 350°F. Line a cupcake pan with paper liners.
Combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt together in a bowl and whisk together.
1 cup all-purpose flour, 1 cup granulated sugar, ½ cup natural unsweetened cocoa powder, 1 teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon salt
Combine the egg, buttermilk, oil, and vanilla in another bowl and whisk together.
1 large egg, ½ cup buttermilk, ½ cup vegetable oil, 1 teaspoon vanilla
Add the wet ingredients to the dry ingredients and mix using a stand mixer until combined.
Add the hot water to the batter and mix again, scraping the sides of the bowl as necessary, until combined. Batter will be thin.
½ cup very hot water
Fill cupcake liners only 1/2 full, then bake for 18-21 minutes, until the tops bounce back when you touch them with a finger or a toothpick comes out with only a few moist crumbs when used to test a cupcake.
Cool completely before frosting with chocolate frosting or your favorite frosting.