The morning before you want to eat these waffles, mix your sourdough starter, water and 110 grams of the self-raising flour in a large bowl to form your "sponge". Cover this with clingfilm and let it rest at room temperature for 12 hours.
During those 12 hours the mix will have begun to ferment creating bubbles throughout. Add the rest of the flour to your bowl and whisk together with the sponge. As it starts to come together in a thick batter, slowly start adding the milk and whisking until you have a smooth, slightly runny consistency. Cover the bowl with clingfilm again and place in the fridge overnight (roughly 12 hours).
When you're ready to make your waffles, lightly brush your waffle maker with vegetable oil and leave to pre-heat according to it's instructions.
In the meantime, add the eggs and vanilla to the batter and whisk well (ideally using an electric whisk), until you have a light and frothy batter.
Once the waffle maker is ready, pour in roughly a cup of batter (the exact amount will depend on the size of your waffle iron) and cook for 3-4 minutes. The waffles are done when they are crisp on the outside and a shade of golden brown.
Enjoy immediately (or follow the instructions in the notes to freeze).