Start by whisking dry ingredients in a big bowl: flour, protein powder, baking powder, and salt. This even spread prevents lumps later.
In another bowl, beat the eggs, milk, oil, vanilla, and egg white until smooth. Pour wet into dry and stir just until combined – a few lumps are fine. Let the batter rest for five minutes; it thickens nicely from the protein.
Heat your waffle iron to medium-high. For Belgian styles, it makes thicker waffles; standard ones give thinner crisps. Grease well with oil spray – protein batter sticks less with a good coat.
Pour 1/4 to 1/2 cup batter per waffle, depending on your iron. Cook until steam slows and edges turn golden, about 3-5 minutes. Keep finished ones warm in a low oven to stay crisp.
Different irons vary, so test the first batch. If they stick, add more grease next time. Stack them high and dig in while hot.