Preheat oven to 350F. Grease a 2 standard sized donut pans (12 donuts total); set aside.
In a medium bowl, combine flour, sugar, cocoa powder, baking soda, baking powder, and salt.
1 ½ cup all-purpose flour, ⅓ cup granulated sugar, 3 tablespoons unsweetened cocoa powder, ½ teaspoon baking soda, ½ teaspoon baking powder, ½ teaspoon salt
Add egg, buttermilk, vegetable oil, and vanilla extract. Whisk until combined. Batter will be on the thicker side.
1 large egg, 1 cup buttermilk, ¼ cup vegetable oil, 1 teaspoon vanilla extract
Spoon batter into a donut pan, filling about 3/4 full.
Bake for about 8-10 minutes. Donuts should spring back when lightly touched.
Allow to cool in the pan for 5 minutes.
Remove from pan, and set on wire cooling rack.
When donuts are almost room temperature, whisk together powdered sugar, cocoa powder, 2 tablespoons buttermilk, and vanilla extra until well-combined.
1 cup powdered sugar, 2 tablespoons unsweetened cocoa powder, 2-3 tablespoons buttermilk, ½ teaspoon vanilla extract
Add additional buttermilk as needed to make a thick glaze.
Dip cooled donuts into glaze, and let set before eating.