Heat waffle iron. Whisk dry ingredients together. Warm milk to room temperature. Whisk yolk with milk and melted butter.
Beat egg white until it just holds 2-inch peak. Add liquid ingredients to dry ingredients in a thin steady stream while mixing gently with a rubber spatula. Fold egg white into batter. Pour batter into waffle iron.
Makes only a few waffles, double for a family of 4.