On the night before breakfast, combine the warm water, sugar and yeast in a large bowl. Let stand for 5-15 minutes, the yeast will proof and turn the mixture bubbly. If you are certain that your yeast is alive you can skip the wait time and move on to the next step.
Add the milk, melted butter, salt, and flour and blend until smooth. A beater or emersion blender makes quick work of this.
Cover the bowl with plastic wrap and let the mixture sit overnight at room temperature. The batter will triple in size and then collapse back down so make certain that the bowl is large enough to accommodate the reaction.
In the morning preheat the waffle iron then beat in the egg yolks and baking soda.
Whisk the egg whites until they hold soft peaks then fold them into the batter.
Oil the waffle iron and pour ½ to ¾ cup of batter (or the amount recommended for your waffle iron) into the iron. Cook until the waffle is golden brown.
Keep the waffles in a warm oven until time to serve. If you place them in a single layer on a rack they will stay crisp.
Notes
Choose a big bowl: The batter will expand overnight, so give it plenty of space to rise.Whip the egg whites: Beat to soft peaks and fold in gently. This step gives the waffles their airy texture.Heat the iron well: A fully preheated waffle iron sears the batter fast, creating a crisp shell.Keep them crisp: Hold finished waffles in a single layer on a wire rack in a 200°F oven. Avoid stacking, which traps steam and softens them.Store or freeze: Leftover waffles keep in the fridge for 2–3 days or can be frozen for up to 3 months. Reheat in the toaster or oven to bring back the crunch.This recipe has been adapted from Rose Levy Beranbaum’s book, The Cake Bible.