Preheat oven to 350 degrees F. Grease two standard 6-cavity donut pans. Set aside.
In a large bowl, sift cocoa powder. Add all-purpose flour, brown sugar, cayenne, baking powder, baking soda, and salt. Whisk together. Make a well in the center of the dry ingredients.
Add eggs and vanilla extract. Use a sturdy spatula to mix eggs into batter. Add buttermilk and melted coconut oil. Stir until incorporated and batter is smooth. Batter will be thick.
Transfer batter to a piping bag fitted with a large round pipping tip. Pipe batter into prepared donut pans, filling cavities 2/3 full. Alternatively, use a spoon to fill donut pan.
Bake for 18-20 minutes until donuts have puffed up and a toothpick inserted in donuts comes out clean. Allow to cool in pan for 3 minutes. Invert donut pan to release donuts. If donuts are sticking to pan, tap edge of donut pan against a sturdy surface to release donuts. Allow donuts to cool on wire rack before dipping in chocolate.