In a large bowl, whisk together flour, sugar, cornstarch, baking powder, baking soda, and salt. Make a well in the center of the bowl,
Add buttermilk, egg yolks (reserve the whites, you will need them in a moment), and vanilla extract to the well. Gently whisk together wet ingredients (avoiding the dry) and drizzle butter into mixture while whisking.
Whisk all wet ingredients together in the well to combine and scramble the egg yolks. Use a spatula to gradually incorporate the rest of the batter until all ingredients are well-combined (but don’t over-mix). There will likely be some small lumps in the batter.
In a separate (completely clean and completely dry) bowl, use an electric mixer to beat egg whites to stiff peaks.
Fold egg whites into batter, stir gently until completely combined,
Let the batter sit for about 5 minutes while your waffle iron preheats.
Once waffle iron is ready, portion batter into hot iron (for my Belgian iron I need about 1 1/4 cups of batter). Cook until golden brown. How long the waffle takes will depend on your iron, for my Presto Belgian iron it takes about 4 minutes. The first waffle is usually a test to judge how much batter to use and how long to cook.
Serve warm, topped with butter and syrup, whipped cream and berries, or powdered sugar!
Making in Advance
This batter can be made and stored, covered, in the refrigerator for two days. Gently stir the ingredients together again before using.