Heat electric griddle to 375 degrees. Whisk flour, cornmeal, baking powder, baking soda, salt, and sugar in a bowl. Add egg yolks, buttermilk, and 4 tablespoons butter; whisk to combine. Batter should have small to medium lumps; fold in egg white.
Heat oven to 175 degrees. Test griddle by sprinkling a few drops of water on it. If water bounces and
spatters off griddle, it is hot enough. Using a pastry brush, brush remaining 1/2 teaspoon of butter onto griddle. Wipe off excess.
Using a 2-ounce ladle (about 1/4 cup) pour batter in pools 2 inches apart. When pancakes bubble on top and are slightly dry around edges, about 2 1/2 minutes, flip. Cook until golden on bottom, about 1 1/2 minutes.
Place on plate in preheated oven until ready to serve.