Heat the oven to 350°F. Prepare a donut pan with non-stick cooking spray. Mix flour, cocoa powder, baking soda, and salt in a large bowl.
1
Combine buttermilk, sugar, egg, butter, and vanilla in a separate bowl.
Mix wet and dry ingredients until just combined. Only fill donut cavities 2/3 to 3/4 full with batter.
2
Bake 8-10 minutes until a toothpick inserted into the donut comes out clean.
Remove from oven and cool for 5 minutes in pan. Remove donuts from pan and cool on wire rack.
3
Mix powdered sugar, cocoa, vanilla, and milk in another bowl. Add one tablespoon of milk to thin the glaze. Glazed donut tops on wire rack to harden. Sprinkle if desired.