Preheat the oven to 200°F. Fit a cooling rack into a baking sheet and set it aside.
In a large bowl, whisk together the flour, cornstarch, salt, baking powder, baking soda and cinnamon (if using). Whisk until combined and set aside.
If you haven't separated your eggs yet, do it now. Mix yolks with the buttermilk, oil, milk and vanilla until combined.
Add whites and cream of tartar (if using) to the bowl of a stand mixer fitted with the whisk attachment (or use a medium bowl and an electric beater.)
Start mixer on medium-low until whites turn foamy, then slowly raise speed to medium-high. Once soft peaks form, gradually stream in sugar. Continue to beat until whites turn firm and glossy, about 2 to 7 minutes, depending on the power of your beater.
Pour liquid ingredients into dry ingredients and stir with a silicone spatula until just blended (a few lumps are fine). Fold in half the beaten egg whites until incorporated to lighten the batter. Then, a little more gently, fold in the rest until fully combined, though a few small streaks of egg white are fine.
Preheat waffle iron (Belgian or American) to the highest setting and oil it lightly. For Belgian-style, pour about a heaping cup of the batter into the preheated waffle iron (for thinner waffles, use about a 1/2 to 3/4 cup). Cook until browned and very crisp (about 5 minutes, depending on your waffle iron).
Transfer the finished waffles in a single layer (do not stack) to the baking tray with the cooling rack and place in oven to keep warm and repeat with the remaining batter. Serve hot with maple syrup or your favorite toppings.