For the overnight sponge, preheat your waffle iron. Combine the starter, flour, sugar, and buttermilk in a large bowl. Stir to mix well. Cover and let rest at cool room temperature for about 8 to 12 hours, or overnight. Timing depends on the temperature of your home.
The next morning, in a small, separate bowl, beat the eggs, and butter. Add to the overnight sponge along with the salt and baking soda. Stir to combine. The batter will bubble, remember sourdough is alive.
Grease your waffle iron. Pour batter into it, and bake according to the manufacturer's instructions.
Serve waffles immediately.