Heat the oven to 200F and heat the waffle iron. Mix the flour, cornstarch, salt, baking powder, and baking soda in a medium bowl. Measure the buttermilk, milk, and vegetable oil in a Pyrex measuring cup; mix in the egg yolk and 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 and cook until the waffle is crisp and nutty brown (follow the manufacturer''s instructions for timing at first and then adjust to your liking). 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.

Variations

Whole-Grain Waffles

Add 1/4 cup wheat germ to the dry ingredients.

Chocolate Chip Waffles

Stir 1/2 cup coarsely chopped chocolate chips (or 1/2 cup mini chocolate chips) into the batter.

Cornmeal Waffles

Substitute 1/2 cup cornmeal for 1/2 cup of the flour (keep the cornstarch).

Cranberry Orange Waffles

Stir 2 tsp. finely grated orange zest and 1/2 cup coarsely chopped dried cranberries into the batter.