Whisk the sourdough starter and milk or buttermilk together in a large bowl.
Whisk in the eggs until smooth.
Stir in the melted butter, sugar or honey, vanilla extract, and salt.
Cover the bowl and rest the batter at room temperature for 8 to 12 hours.
Preheat the waffle iron until fully hot.
Sprinkle the baking soda over the batter.
Stir gently until the baking soda disappears.
Let the batter rest for 2 minutes.
Lightly grease the waffle iron.
Pour batter into the center and close the lid.
Cook for 3 to 5 minutes, depending on your waffle maker.
Wait for steam to slow before opening the lid.
Move finished waffles to a wire rack instead of stacking on a plate.
Keep waffles warm in a 200°F oven on a rack while you finish the batch.