The night before, whisk yeast, flour, sugar and salt in a large bowl; whisk in butter, then milk. The mixture will be creamy and loose. Cover with plastic wrap; let stand at room temperature overnight (batter will expand).
When ready to cook, brush the waffle iron lightly with oil and heat it according to manufacturers instructions. Separate the egg and stir the yolks into the batter. Beat the whites until they hold soft peaks. Stir them gently into the batter. Cut bacon crosswise into 4 pieces.
Add 1/3 cup of batter for each 4-inch-square waffle; place 4 bacon pieces on each square. Bake 6 to 7 minutes or until nicely browned. Repeat with remaining batter and bacon.
Place waffles on ovenproof serving platter in 250 degrees F oven until all batches are cooked. To prepare ahead, bake waffles, let cool. Wrap loosely in plastic wrap; refrigerate for up to a day. Reheat in a single layer at 250 degrees F until crisped, 10 minutes. Serve with syrup.