Gently warm the milk until just lukewarm to the touch and melt the butter; set both aside to cool slightly so they don’t cook the eggs.
In a large bowl, whisk together the flour, baking powder, salt, cinnamon, and nutmeg.
Separate the eggs. In a medium bowl, whisk the egg yolks with the granulated sugar (reserve 1 teaspoon of the sugar for the egg whites), vanilla, maple syrup, and the lukewarm milk. Whisk in the melted butter.
Pour the yolk mixture into the dry ingredients and whisk just until combined; a few small lumps are fine. Cover and let the batter rest for 10 minutes to hydrate.
While the batter rests, preheat the waffle iron to a medium setting and set your oven to 200°F with a wire rack on a sheet pan inside for holding finished waffles.
In a clean mixing bowl with clean beaters, whip the egg whites with the reserved 1 teaspoon sugar to soft, glossy peaks (they should curl slightly at the tip when you lift the whisk).
Using a wide spatula, fold the whites into the rested batter in two additions, stopping as soon as the streaks just disappear to keep the batter airy.
Lightly brush the hot waffle iron with melted butter or oil. Add batter (about 1/2 to 2/3 cup for a Belgian iron; adjust for your model), close, and cook until deeply golden and the steam mostly subsides, 3 1/2 to 4 minutes per waffle.
Transfer waffles to the warm oven, placing them directly on the rack to stay crisp while you finish the batch. Repeat with remaining batter, brushing the iron only as needed.
Serve hot with your favorite toppings—think butter, warm syrup, fresh fruit, or a dusting of powdered sugar.