1. In a large mixing bowl, combine the sourdough discard, milk, eggs, melted butter, and vanilla extract (if using). Whisk thoroughly until everything is well combined and smooth.

2. To the wet mixture, add the all-purpose flour, sugar (if using), and salt. Whisk continuously until the batter is smooth and free of lumps. Do not overmix; just combine until uniform. The consistency should be thin, like heavy cream.

3. Optional but Recommended: Let the batter rest for 5-10 minutes at room temperature to help the flour fully hydrate, resulting in more tender crepes. If you are in a hurry, you can skip this step.

4. Place a non-stick crepe pan or a flat-bottomed skillet (8-10 inches) over medium heat. Lightly brush or wipe the pan with a little melted butter. You want it hot enough that a drop of water sizzles instantly, but not smoking.

5. Pour about 1/4 cup of batter into the center of the hot pan. Immediately lift the pan and tilt and swirl it gently to spread the batter evenly across the entire bottom surface in a thin layer. Work quickly!

6. Cook for 1-2 minutes, or until the edges start to look dry and slightly crispy, and the bottom is lightly golden brown. You should see small bubbles forming on the surface.

7. Carefully slide a thin spatula under the crepe and flip it over. Cook for another 30-60 seconds on the second side, just until it is lightly golden.

8. Transfer the cooked crepe to a plate. Lightly butter the pan again if needed before pouring the next batch of batter. Continue with the remaining batter, stacking the cooked crepes on top of each other to keep them warm.