Preheat a waffle iron, greasing as needed to prevent sticking.
In a medium mixing bowl, stir together the flour, cornstarch, salt, baking powder, baking soda, and sugar. Pour in the buttermilk, milk, melted butter, egg yolk, and vanilla. Whisk to combine.
In a separate bowl, beat the egg white until soft peaks form. Fold the egg white into the waffle batter.
Scoop the batter into the waffle iron. Cook until dark golden brown and crisp, about 4-5 minutes, depending on the waffle iron. Serve promptly with warm maple syrup. Repeat with remaining batter.