Preheat your waffle iron according to the manufacturers instructions. Preheat your oven to 200ºF to keep the waffles warm and crisp until serving.
In a large bowl, whisk the flour, cocoa powder, sugar, baking powder, baking soda and salt together. In a large glass measuring cup, add the buttermilk, vanilla, eggs and melted butter and whisk well. Pour the wet ingredients into the dry ingredients and mix until the batter is just combined.
Use cooking spray on the inside of the waffle iron, then pour about 1/2 cup of batter into the center of the iron. The amount of batter will depend on your waffle iron. Cook for 3-5 minutes, or until crispy on the outside and done all the way through.