In a large bowl, whisk together the flour, sugar, baking powder, and salt. Break up any visible lumps so the leavening is evenly distributed.

In a separate medium bowl, whisk the egg yolks, milk, melted butter, and vanilla until smooth. The butter should be melted but only slightly warm so it does not scramble the yolks.

Pour the wet mixture into the dry ingredients and whisk gently just until combined. The batter should be thick but pourable. A few small lumps are preferable to overmixing.

In a clean bowl, beat the egg whites with an electric mixer on medium-high speed until they form soft to medium peaks. The whites should hold their shape when the beaters are lifted but still look smooth rather than dry.

Add about one-third of the whipped egg whites to the batter and fold gently to lighten it. Fold in the remaining whites in two additions, using a rubber spatula and stopping as soon as no large streaks of white remain.

Let the batter rest for 10 minutes at room temperature. It should become slightly thicker while remaining easy to scoop or pour.

Preheat the Belgian-style waffle maker according to the manufacturer's instructions. If the plates are not reliably nonstick, lightly grease them before the first waffle.

Add enough batter to cover the waffle plates without overflowing; for many deep Belgian-style makers, about 1/2 to 3/4 cup (120 to 180 ml) per waffle is appropriate, but follow your appliance's capacity. Close the lid and cook until the waffle is deeply golden brown and releases easily, usually about 4 to 5 minutes.

The waffle should feel crisp around the edges and firm enough to lift without bending excessively. If it is pale or soft, continue cooking in short intervals until the exterior is properly browned.

Transfer each waffle directly to a wire rack: Do not stack the waffles while hot, because trapped steam will soften the exterior.

Repeat with the remaining batter, lightly greasing the waffle plates between batches only if needed.

Serve the Belgian Waffles immediately with optional maple syrup, fresh berries, whipped cream, or powdered sugar.