1. Whisk the Wet Ingredients: In a large mixing bowl, combine the sourdough discard, milk, eggs, melted butter, optional sugar, optional vanilla extract, and salt. Whisk vigorously until all the ingredients are well combined and smooth. The mixture should be fairly liquid.

2. Incorporate the Flour: Gradually add the all-purpose flour to the wet ingredients, whisking continuously to prevent lumps. Continue whisking until the batter is smooth and resembles a thin pancake batter. Don’t overmix; a few small lumps are okay and will often disappear during the resting period.

3. Allow the Batter to Rest: Cover the bowl with plastic wrap or a clean kitchen towel and let the batter rest at room temperature for at least 15-20 minutes. This resting period allows the flour to fully hydrate, resulting in a smoother batter and more tender crepes. If you have more time, you can even rest it in the refrigerator for up to an hour.

4. Heat the Crepe Pan: Place a non-stick crepe pan or a shallow frying pan over medium heat. Lightly brush the pan with a small amount of melted butter or oil. The pan is ready when a drop of water sizzles and evaporates quickly.

5. Pour and Swirl the Batter: Pour about 1/4 cup of batter into the center of the hot pan. Immediately lift the pan and gently tilt and swirl it in a circular motion to evenly spread the batter into a thin circle across the bottom of the pan. Work quickly to ensure an even, thin crepe.

6. Cook Until Golden: Cook the crepe for about 1-2 minutes on the first side, or until the edges start to look dry and slightly crispy, and the bottom is lightly golden brown. You can gently lift an edge with a spatula to check for color.

7. Flip and Finish Cooking: Carefully slide a thin spatula under the crepe and flip it over. Cook for another 30 seconds to 1 minute on the second side, or until lightly browned.

8. Repeat and Stack: Transfer the cooked crepe to a plate. Continue with the remaining batter, lightly greasing the pan between each crepe if needed. Stack the cooked crepes on top of each other; the residual heat will keep them warm and pliable.