Place the sourdough starter in a large bowl and whisk in the buttermilk, flour and sugar until combined. Cover the bowl and allow it to rest overnight at room temperature.
The next morning: Waffle batter
When you are ready to cook, whisk the egg white until soft peaks; set aside.
In another bowl, whisk egg yolk, melted butter, oil and the vanilla extract together, then add to the rested sponge. Fold in the egg white.
Add the salt and the baking soda to the batter and mix to combine.
Heat your waffle iron according to manufacturer instructions. I do medium heat.
Pour some of the batter onto a preheated greased waffle iron and cook until the waffle is brown and crisp, then repeat with remaining batter.
Notes
To make Belgium waffles: sprinkle pearl sugar over the waffle batter before closing the lid.For wild blueberry waffles you can either fold ½ cup fresh or frozen wild blueberries into the batter or just sprinkle over the top before closing the lid.Buttermilk substitutions:1. Place 1 tablespoon of lemon juice or white vinegar in a measuring cup and add enough milk to make 1 cup...or2. Combine ½ cup of milk with enough plain Greek yogurt to measure 1 cup.Lemon oil substitution: 2 tablespoons of any flavorless oil plus grated zest of 1 lemon, but lemon zest is optional.Pro tip: I like to sift in the baking soda to prevent lumps.