In a large mixing bowl, whisk together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.
In another bowl, mix the milk, vegetable oil, vanilla extract, and egg until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Lightly grease the waffle iron with cooking spray or oil. Pour the batter onto the preheated waffle iron and cook according to the manufacturer's instructions until the waffles are crispy and cooked through.