Combine overnight sponge ingredients. Mix well and let sit overnight. (I choose to let the milk sit at room temperature overnight to help it ferment. If you are uncomfortable with that, feel free to refrigerate it overnight.)
In the morning, add the remaining ingredients, mix well, and cook your waffles on a waffle iron.
The batter can be stored in the fridge for up to 3 days.
Notes
This recipe was adapted from my friend and sourdough expert extraordinaire, @simplelifebykels