For sour milk place 1 Tbsp. lemon juice or vinegar in a liquid measuring cup. Add enough milk to make 1 cup. Stir; let stand 5 minutes before using.To make ahead, let each waffle cool, then layer between sheets of waxed paper. Freeze in an airtight container up to 3 months. To reheat, unwrap waffle and toast in the toaster until heated through.