Preheat a regular waffle iron according to manufacturer’s instructions.

Beat milk, egg, sugar, and vanilla together in a large bowl until well combined. Add baking mix and beat until combined; some small lumps are okay. Transfer batter to a liquid measure.

Spray the top and bottom of the preheated waffle iron with cooking spray. Slowly pour in enough batter to cover the bottom of the iron, being careful not to let it overflow.

Close the lid and cook until waffle is golden brown and the iron stops steaming, about 5 minutes. Repeat to cook remaining waffles.