Whisk the flours, baking powder, salt, and sugar together in a medium bowl.
Add the milk, eggs, and vanilla extract to the bowl. Stir with a fork until just combined.
Pour in the melted butter. Gently fold it in just until the batter looks combined—don’t worry if there are a few lumps.
Let the batter rest for 5 minutes to develop (this brief rest allows the baking powder to activate).
Meanwhile, heat your waffle iron. Pour, or ladle, enough batter to cover about 2/3 of the surface (the rest will spread once you close the top). Cook according to the manufacturer’s directions. A sure sign of doneness is once you see all the steam has stopped shooting out the sides of the waffle iron.