Preheat your Belgian waffle maker to medium-high. Set up a wire cooling rack over a baking sheet near the iron to keep finished waffles crispy.
In a large bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and fine salt until evenly combined.
Carefully separate the eggs, placing the yolks in one small bowl and the whites in a clean, dry medium bowl.
In a medium bowl, whisk the egg yolks, whole milk (or buttermilk), melted butter, vanilla extract, and vinegar (if using) until well combined.
Pour the wet mixture into the dry ingredients and stir gently just until combined. Do not overmix; a few small lumps are fine.
Using an electric hand mixer, beat the egg whites on medium speed until foamy, then increase to high speed and beat until stiff, glossy peaks form (2-4 minutes).
Fold one-third of the whipped egg whites into the batter to lighten it. Gently fold in the remaining whites until just a few streaks remain.
Lightly grease the preheated waffle iron. Pour about ¾ to 1 cup of batter onto the center of the lower plate. Close the lid and cook for 4-6 minutes, until deep golden brown and steam has mostly stopped escaping.
Carefully remove the waffle and place it on the wire cooling rack. Repeat with the remaining batter, greasing the iron between each waffle.