Preheat the oven to 350°F.
In a large bowl whisk the cocoa powder, flour, salt, baking powder, and baking soda. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment add the eggs, granulated sugar, brown sugar, oil, and vanilla. Mix on medium speed for 1 minute or until thoroughly combined.
With the mixer on low, alternate adding the dry ingredients with the buttermilk starting and ending with the dry ingredients. (Dry, buttermilk, dry, buttermilk, dry). Don't overmix.
Line a regular sized muffin tin with cupcake liners. Fill the liners half full. If you fill them over halfway, they will spill over! Don't do it. Only fill half full.
Bake for 18-20 minutes or until a toothpick inserted in the center comes out clean.
Let cool in pan for 5 minutes, then remove to a wire rack to cool completely.
Frost with chocolate buttercream frosting.