In a large bowl, whisk the cocoa powder, flour, baking soda, baking powder, and salt together until thoroughly combined. Set aside. In a medium bowl, whisk the eggs, sugar, brown sugar, oil, and vanilla together until completely smooth. Pour half of the wet ingredients into the dry ingredients. Then half of the buttermilk and half of your beer. Gently stir a few times. Repeat with the remaining wet ingredients, buttermilk and beer. Stir until just combined; do not overmix. The batter won’t be very thick.