Preheat your waffle iron on a medium-high setting. While it heats, place a cooling rack over a baking sheet in a low oven (about 100°C). This keeps your finished waffles warm and crisp without steaming them.

Whisk the dry ingredients together in a large bowl: the flour, sugar, baking powder, salt, and cinnamon if using. You’ll see the pale, sandy mixture look light and aerated — that’s a good sign.

In a separate bowl, beat the egg yolks with the milk, oil, and vanilla extract. The mixture will turn a pale, creamy yellow and smell sweetly of vanilla. Pour this into the dry ingredients and stir gently with a wooden spoon until just combined. A few small lumps are fine — overmixing will develop the gluten and make the waffles tough. The batter will look thick and slightly lumpy, like a very thick pancake batter.

In a clean, dry bowl, whisk the egg whites with the cream of tartar until stiff peaks form. You’ll know they’re ready when you lift the whisk and the peaks stand upright without collapsing. The whites should look glossy and white, not grainy or dry.

Fold the egg whites into the batter in two additions. Use a gentle, cutting motion with a rubber spatula. The batter will transform from thick and dense to light and airy, almost like a soufflé mixture. You’ll see streaks of white disappear as you fold — stop as soon as no white streaks remain, even if a few lumps persist.

Let the batter rest for 5 minutes. During this time, the baking powder activates and the gluten relaxes. You’ll notice tiny bubbles forming on the surface, which promise a tender crumb.

Grease the hot waffle iron with a little butter, then pour in enough batter to cover the grids without overflowing — usually about ¾ cup for a standard square iron. Close the lid and cook for 4-5 minutes. You’ll see steam escaping initially, then it will slow. When the waffle is ready, the iron will release it easily, and the waffle will be deep golden brown with a crisp, crackling sound when you touch it.

Transfer the cooked waffle to the rack in the oven to stay warm and crisp. Repeat with the remaining batter, greasing the iron between each batch. The smell of toasted butter and vanilla will fill your kitchen.