In a large bowl, whisk together the sourdough starter or discard, flour, melted butter, eggs, milk, maple syrup, salt and vanilla. The maple syrup is optional here, I sometimes don't add it if I know I'll be adding enough sweetness with my filling/toppings. Whisk until the batter is completely smooth. An immersion blender can be used if you can't get out some of the lumps.
Allow the batter to ferment in the fridge overnight, or 12-24 hours. The batter might get a little thicker as it sits, you want it to be the consistency of heavy cream. If it's thicker than that, thin it out with a little extra milk or water.
The next day, heat a non-stick skillet over medium-low heat.
Grease the bottom of the pan with a little butter.
Using a measuring cup, pour ¼ cup of batter onto the skillet. Pick up the skillet and swirl the batter in a circular motion until it spreads into a very thin, even layer.
Let it cook until the first side is golden brown with crispy edges, 60-90 seconds. Using a large flat spatula, flip the crepe onto the second side and let it cook for another minute.
Remove from skillet and stack the crepes on a platter.