Warm the milk to 105–110°F; whisk in the yeast and sugar, and let stand until foamy, 5–10 minutes.
In a large bowl, whisk the flour and salt until evenly combined.
Stir the foamy mixture and butter into the dry ingredients until smooth; cover and let rise in a warm spot until doubled, 60–75 minutes.
Separate the eggs; beat the whites to soft peaks, whisk the yolks into the risen batter, then gently fold in the whites until no streaks remain. Let the batter rest 10 minutes.
Heat a Belgian waffle iron to medium-high and lightly brush with oil.
Pour 0.5–0.75 cup batter per waffle; close and cook until deep golden and crisp and steam largely subsides, 3–5 minutes per waffle. Repeat with remaining batter, brushing the iron lightly as needed.