Instructions

1. Place 1 1/4 cups whole milk and 1 stick cut-up unsalted butter in a medium microwave-safe bowl or large measuring cup. Microwave in 30-second intervals, stirring after each, until the butter is just melted and the mixture is warm but not hot, 1 to 2 minutes total. (Alternatively, heat in a small saucepan over medium-low heat.)
2. Whisk 2 cups all-purpose flour, 1/2 cup powdered sugar, 1 tablespoon baking powder, and 1 teaspoon kosher salt together in a large bowl. Make a well in the center of the flour mixture. Pour the butter mixture into the well, then add 3 large egg yolks and 2 teaspoons vanilla extract. Whisk the wet ingredients together in the center of the bowl until smooth. Then, working from the inside out, gradually whisk in the dry ingredients until thick and mostly smooth. A few small lumps are fine, but avoid large pockets of dry ingredients.
3. Beat 3 large egg whites in a medium bowl with an electric mixer on medium speed or by hand until stiff, glossy peaks form, about 2 minutes with the mixer. Fold about one-third of the egg whites into the batter with a flexible spatula to lighten it. Fold in the remaining whites until no streaks remain.
4. Heat the oven to 250°F. Fit a wire rack onto a rimmed baking sheet. Heat a Belgian waffle maker according to the manufacturer's instructions. A drop of water should sizzle and evaporate when it’s hot enough.
5. Lightly brush the top and bottom of the iron with melted butter, or coat lightly with cooking spray. Add about 3/4 to 1 cup of the batter (depending on your machine) to the waffle iron. Gently spread until the surface is almost totally covered. Close the lid and cook until well-browned and crisp, 5 to 7 minutes or according to the manufacturer's instructions.
6. Serve immediately, or transfer the waffle to the wire rack and keep warm in the oven. Repeat making more waffles with the remaining batter, greasing the waffle iron before each waffle.