Preheat waffle iron. Spray with non-stick cooking spray right before pouring batter inside.
In a large bowl, combine the flour, cocoa powder, sugar, baking powder, and salt. Use the back of a spoon to create a well in the center. Set aside.
In a small bowl, whisk together eggs and milk. Add in butter and vanilla and whisk to combine.
Pour the wet ingredients into the well of the flour mixture and whisk until blended. Be careful not to overmix, as the batter should be lumpy.
Scoop the batter into the preheated waffle iron and cook until the waffles are browned and slightly crispy.