Heat the oven to 200°F and heat up your waffle iron. Mix the flour, cornstarch, salt, baking powder, and baking soda in a medium bowl. In a measuring cup or small bowl, mix the buttermilk, milk, vegetable oil and egg yolk and set aside.
In another bowl, beat the egg white to almost soft peaks. This part sucks (sorry). 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 and cook until the waffle is crisp. Remove the waffles and place on a baking sheet in your oven to keep warm. Repeat with the remaining batter, holding the waffles in the oven (don’t stack them). When all the waffles are cooked, serve immediately.