Heat the milk to lukewarm (about 110 degrees) in a small saucepan over medium-low heat or by microwaving for 1 minute. Pour into a large bowl and stir in the sugar, then the yeast. Let stand until the top becomes bubbly, about 10 minutes.
Meanwhile, beat the egg whites using an electric mixer with the whisk attachment on medium speed or whisk by hand until medium peaks form.
Whisk the butter into yeast mixture, then whisk in the salt and egg yolks. While whisking, add the flour bit by bit. Whisk until smooth. Add one-third of the egg whites and fold gently until incorporated. Repeat two more times, folding just until no white streaks remain after each addition. Cover the bowl with plastic wrap and refrigerate for at least 1 hour and up to 12 hours.
Heat a waffle iron to medium heat. If the batter has rested more than 1 hour and has puffed a lot, gently fold it to deflate. Scoop and cook the batter according to your waffle iron’s instructions. Serve hot with syrup, whipped cream, berries or other toppings of your choice.
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