In medium bowl, combine flour, baking powder, sugar, and salt. Whisk until well combined.
In separate medium bowl, whisk eggs and milk until combined. Add sourdough starter and whisk until completely blended. Add flour mixture and butter and stir until combined. Depending on thickness of starter, you might need to adjust batter. It should be the consistency of pancake batter. Add flour or milk as needed to reach that consistency.
Cook in waffle iron according to manufacturer’s directions.
Adapted from Serious Eats.