Preheat the waffle iron.
Whisk together flour, sugar, baking powder, baking soda, and salt in a large bowl.
Whisk together buttermilk, melted butter, and eggs in another bowl, then whisk into flour mixture until just combined.
Spray waffle iron with cooking spray and pour 1/2 cup of batter into the waffle iron.
Cook waffles until golden and cooked through, about 3 minutes.
Transfer to a baking sheet and into the oven set to 250 degrees while you make the rest of the waffles.