Combine flour and salt in a bowl; form a well in the center. Pour in egg. Using one hand, with fingers spread wide, mix egg into flour while pouring in 1 1⁄2 cups cool water. Stir and slap batter upward with your right hand, scooping it up from the bottom. Continue with increasing speed and force, lifting and slapping the batter to make a hollow, spanking noise with the impact. Continue this process until batter is smooth and elastic, 3–4 minutes. ("The more you work it, the easier it will be to work with afterward," says Nahon.) Transfer batter to a medium bowl and pour 2 tbsp. water evenly over the top (this protects the batter from drying out). Cover surface with plastic wrap, tucking in the edges. Cover and refrigerate overnight.
Stir milk and 2⁄3 cup water into batter. Heat a well-seasoned 10 1⁄4" crepe pan over medium heat and brush with a bit of butter, wiping off any excess. Pour about 1⁄4 cup batter onto pan, swirling quickly to spread batter out to edges. Cook until light golden and just crisp, about 2 minutes. Using a spatula, flip and cook until light golden on the second side, about 2 minutes more. Repeat with remaining batter. Spread a dab of the butter onto each crepe and serve hot with Cider-Spiked Onion Confit or Scallops in White Wine Cream Sauce