Preheat your waffle maker according to the manufacturer's instructions.
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking powder, and salt until well combined.
In another bowl, mix the melted butter, eggs, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. If using, fold in the chocolate chips.
Lightly grease the waffle maker with cooking spray or a small amount of melted butter.
Pour enough batter into the waffle maker to cover the surface (about 1/2 to 3/4 cup, depending on the size of your waffle maker). Close the lid and cook according to the manufacturer's instructions, usually about 3-5 minutes, until the waffle is cooked through and crispy.
Carefully remove the waffle and repeat with the remaining batter.