Combine the starter, milk, and flour in a medium bowl and cover with plastic wrap. Rest for 8 hours at room temperature, until the batter is light, full of bubbles, and roughly doubled in size. (If you'd like to make these for breakfast, you can refrigerate the batter overnight after this 8-hour rest.)
Stir the egg yolk, butter, sugar and salt into the starter mixture until just combined and set aside.
Beat the egg white and cream of tartar with a hand mixer on high until stiff peaks form, 1 to 2 minutes, then fold the egg white into the batter using as few folds as possible to keep it from deflating (see Cook's Note). Rest the batter at room temperature for 20 minutes.
Heat a Belgian waffle iron on medium heat. When hot, lightly coat both plates with nonstick spray. Ladle the recommended amount of waffle batter onto the iron according to the manufacturer's recommendations and cook until crisp and golden brown. Serve immediately or cool on a rack and wrap in plastic wrap followed by foil and freeze for up to 3 months.