Heat the oven to 200°F and heat up the waffle iron.
Mix the flour, cornstarch, salt, cinnamon, nutmeg, baking powder, and baking soda in a medium bowl. Measure the buttermilk, milk, and vegetable oil in a measuring cup and stir in the egg yolk. Set aside.
In another bowl, beat the egg white almost to soft peaks. Sprinkle in the sugar and continue to beat until the peaks are firm and glossy. Beat in the vanilla.
Pour the buttermilk mixture into the dry ingredients and whisk until just mixed. Drop the whipped egg white onto the batter in dollops and fold in with a spatula until just incorporated.
Pour the batter onto the hot waffle iron (My Belgian waffle maker takes 2 1/2 ice cream scoops of batter, but your mileage may vary) and cook until the waffle is crisp and brown.
Set the waffle directly on the oven rack to keep it warm and crisp. Repeat with the remaining batter, holding the waffles in the oven (don't stack them). When all the waffles are cooked, serve immediately.
This recipe makes 4-5 waffles in my Belgian waffle maker. The number of waffles may vary depending on your waffle maker.