Preheat your waffle iron to medium-high. Lightly oil or spray it so the first waffle behaves. The first one is often the tester. That’s normal.
In a large bowl, whisk the dry ingredients: flour, cornstarch, sugar, baking powder, baking soda, cinnamon, nutmeg, and salt.
In a second bowl, whisk the wet ingredients: buttermilk, eggs, vanilla, and melted butter.
Pour the wet into the dry. Whisk until just combined. A few small lumps are fine. Fold in the grated apple. If the batter looks very thick, add a splash of milk. If it looks thin, sprinkle in a tablespoon of flour.
Let the batter rest for 5 minutes while the iron finishes heating. This quick pause helps the flour hydrate and gives you a better waffle.
Cook the waffles: add batter to the hot iron (amount depends on your model), close, and cook until deeply golden and the steam has mostly subsided, 3 to 5 minutes. Resist the urge to peek too soon.
Keep finished waffles warm and crisp on a wire rack in a 200°F or 95°C oven. Try not to stack them, or they’ll steam.
Serve with butter and maple syrup. If you want a little extra, quickly sauté sliced apples in a small skillet with a teaspoon of butter, a sprinkle of cinnamon, and a drizzle of syrup until glossy.