1. Heat the waffle iron according to the manufacturer's instructions.
2. Meanwhile, whisk together the flour, cornmeal (if using), salt, and baking soda in a medium bowl. In a separate bowl, whisk together the egg yolk, butter, and then the buttermilk.
3. Beat the egg white and cream of tartar with a hand-held mixer on high speed until the stiff peaks form, 2-3 minutes.
4. Add liquid ingredients to dry ingredients in a thin steady stream while gently mixing with a rubber spatula; be careful not to add liquid faster than you can incorporate it. Toward end of mixing, use a folding motion to incorporate ingredients; gently fold egg white into batter.
5. Following the manufacturer's instructions, spread the appropriate amount of batter onto the waffle iron and cook until golden brown, about 3 1/2 minutes. Repeat with the remaining batter, serving the waffles immediately or holding them in a 200-degree oven (see the note above) until all are cooked.