Preheat waffle iron: Heat to medium-high (about 375–400°F equivalent).
Mix dry ingredients: Whisk flour, sugar, baking powder, baking soda, salt, and optional cornstarch in a large bowl.
Combine wet ingredients: In another bowl whisk egg yolks, buttermilk, melted butter, and vanilla.
Mix batter: Pour wet into dry ingredients and stir until just combined — don’t over mix.
Whip egg whites: In a clean bowl, beat egg whites to soft peaks, then gently fold into batter.
Rest batter: Let the batter sit 5–10 minutes to hydrate and activate leaveners.
Cook waffles: Lightly grease the iron and pour batter (about ¾–1 cup). Close lid and cook until golden and steam reduces.
Keep warm: Place cooked waffles on a wire rack in a 200°F oven if not serving immediately.