Preheat oven to 350° and prepare donut pan by spraying with non-stick cooking spray. Set aside.
In a large bowl, whisk together the flour, cocoa powder, brown sugar, baking soda, baking powder, and salt. Set aside.
In a medium-sized bowl, add eggs, buttermilk, vanilla, and cooled melted butter. Whisk until well combined.
Pour the wet mixture into the dry ingredients and mix until fully incorporated.
Transfer donut batter into a piping bag or large Ziploc bag and pipe into prepared donut pans. Pipe batter until it fills the donut pan halfway.
Bake in preheated oven for 14-16 minutes or until a toothpick inserted in the center comes out clean.
Remove from oven and let cool in pans for 3-5 minutes. Remove from pans and set aside for icing.