Combine the flour, milk, water, eggs, butter, and salt in a blender, and blend until the mixture is smooth—only about 10 seconds. The batter should be much thinner than pancake batter—closer to the consistency of heavy cream. Let the batter rest for at least an hour, and up to overnight. (Refrigerate if you are not going to use it for more than an hour.)
After resting the batter, give it a gentle whisking to recombine. Then heat a crêpe pan or nonstick skillet on medium, cut off a sliver of butter, and swirl it around the pan. When the butter melts, use a paper towel to coat the pan evenly.
Pour about ¼ cup of batter directly in the center of the pan, then quickly lift the pan up and roll your wrist around as you hold it, so that the batter spreads out in a thin even layer over the pan. Cook until you see the lacy edges begin to brown, then use a non-scratch spatula to flip your crêpe and cook the other side.
Keep an eye on your heat—you want the crêpes to cook for about 1-2 minutes on the first side, and 30-60 seconds on the second side. Repeat with the rest of your crêpes, including buttering the pan between each.
As you finish your crêpes, stack them on a plate separated by squares of wax paper or parchment paper.
Notes
If you don’t have a blender or don’t want to use one, just whisk the crêpe ingredients together in a mixing bowl until well-combined (about a minute). If your batter has a layer of foam on top from being aggressively blended, just skim it off before making your crêpes. (You can also just leave it, but it will make some of those first crêpes a little bubbly.)Adjust the heat under the pan if your crêpes start to cook too quickly (watch for the lacy edges starting to brown immediately). The pan will start to hold onto the heat the longer you use it, so you may need to turn the burner from medium heat to medium-low heat as you go. Note that if your pan is too hot, your crêpe batter won’t spread out enough and can lead to crêpes that are too thick.If you keep your crêpe batter in the refrigerator overnight, it will separate as it sits. Just stir it gently to bring the batter back together.