Preheat your waffle iron according to the manufacturer’s instructions and spray generously with nonstick cooking spray. In a large bowl, whisk together the flour, sugar, cornstarch, baking powder, and salt until evenly combined. In a medium bowl, beat the egg whites until stiff peaks form. In another bowl, whisk the egg yolks with milk, vegetable oil, and vanilla extract until smooth. Add the wet mixture to the dry ingredients and stir just until combined. Gently fold in the whipped egg whites until the batter is light and airy. Pour ½ to 1 cup of batter into the hot waffle iron and cook for 5 to 8 minutes until golden brown and crisp. Transfer cooked waffles to a 300°F (150°C) oven to keep warm until ready to serve.