1. Preheat the oven to 225°F | 110°C. 

Make the Waffle Batter

1. Combine the flours, oats, cornstarch, salt, baking powder, and baking soda in a large bowl. In a separate bowl, whisk together the buttermilk, melted butter, and egg yolks. In a third bowl, using an egg beater or an electric mixer, beat the egg whites into stiff peaks.

2. Heat the waffle maker, and when it is ready, add the buttermilk mixture to the dry ingredients and stir until the mixture just starts to come together. Dollop the egg whites across the top of the batter and fold until uniform, using a few strokes as possible.

Cook the Waffles

1. Use a scoop to ladle the batter into your waffle iron and cook until deeply golden and crisp. Transfer to the warm oven while you make the remaining waffles--the dry heat of the oven helps them set a bit. Any leftover batter will keep for a day or so, refrigerated.