How to Make It
Preheat a waffle iron according to manufacturer’s instructions. Preheat oven to 250°F. Set a wire rack in a rimmed baking sheet and place in oven.
Melt butter in a medium skillet over medium, stirring occasionally, until it browns and smells nutty, 4 to 6 minutes. Pour into a bowl; let cool for 5 minutes.
Meanwhile, beat egg whites with an electric mixer on high until soft peaks form, 2 to 3 minutes. Gradually beat in sugar.
Whisk flour, baking powder, and salt in a large bowl. Whisk reserved egg yolks, milk, vanilla, and cooled brown butter in a separate large bowl. Stir egg yolk mixture into flour mixture until evenly incorporated. Fold in egg whites until just combined.
Brush waffle iron with melted butter or coat with nonstick spray. Spoon batter into waffle iron (about 1/3 cup batter per square waffle or ½ cup batter per round waffle). Cook according to manufacturer’s instructions until golden brown, 4 to 5 minutes. Transfer waffle to baking sheet in oven to keep warm. Repeat with remaining batter, buttering or spraying iron between batches. Serve with butter and maple syrup.
Let waffles cool completely. Transfer to sealable plastic bags and freeze for up to 2 months. To reheat, place frozen waffles directly on oven rack at 350°F until warm, about 10 minutes (or reheat in a toaster).