Heat waffle iron. Whisk dry ingredients together in a medium bowl. Whisk yolk with milk and butter. Beat egg white until it just holds a 2-inch peak. 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. Spread appropriate amount of batter onto waffle iron. Following manufacturer's instructions, cook waffle until golden brown, 2 to 5 minutes. Serve immediately. (You can keep waffles warm on a wire rack in a 200-degree oven for up to 5 minutes.) Makes 3 to 4 waffles. Notes : "If you're out of buttermilk, try this sweet-milk variation. By making your own baking powder using baking soda and cream of tartar and by cutting back on the quantity of milk, you can make a thick, quite respectable batter. The result is a waffle with a crisp crust and a moist interior." -- Christopher Kimball, Cook's