makes 4 or 5 Belgian waffles about 6-inch diameter
Night before: Mix starter, flour, milk, and sugar in a large bowl with enough capacity for batter to rise at least 50% and up to 100%. Cover with plastic wrap and leave out at room temperature until the next morning.
Morning: Stir the melted butter, egg, salt, and baking soda into the overnight batter. Pour ½ cup batter into preheated waffle iron and cook until waffle is golden and crisp.