Step 1Heat 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.
Step 2Meanwhile, beat the egg whites using an electric mixer with the whisk attachment on medium speed or whisk by hand until medium peaks form.
Step 3Whisk 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.
Step 4Heat 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.
Pancake Puffs: Omit the waffle iron. Heat a nonstick skillet over medium-low heat. Coat the inside of a 3-inch-round (2-inch-high) metal biscuit cutter with nonstick cooking spray. Set in the skillet and lightly spray the pan inside the cutter. Add ⅓ cup batter, cover the skillet with a lid, and cook until the bottom is golden brown and the top is almost dry, about 5 minutes. Slide a very thin spatula under the pancake and cutter, then carefully flip both together. Cook until the other side is golden brown and the pancake bounces back when you poke its top, 3 to 5 minutes. Slide the spatula under the pancake and cutter and transfer to a plate. Using a kitchen towel or pot holders, gently wiggle the cutter off the pancake. Repeat with the remaining batter. Serve immediately.