Sift flour and salt into a large bowl. Whisk together milk and eggs in a medium bowl. Pour milk mixture into flour mixture, whisking to combine. Whisk in butter. Strain mixture into a medium bowl, and refrigerate for at least 2 hours (or up to 1 day). Batter should be the consistency of heavy cream; add more milk if needed.
Heat an 8- or 12-inch nonstick skillet over medium heat, and brush with butter. Ladle or pour 3 tablespoons batter (for small crepes) or 1/3 cup batter (for large crepes) into pan, turning and tilting skillet to coat bottom evenly with batter. Cook until top of crepe appears set, bottom is firm and golden brown in spots, and center is lifted by pockets of air, about 1 minute.
Run a spatula around edge of crepe to loosen. Slip spatula under crepe, and gently flip in one swift gesture. (If it doesn't land quite right, that's okay; use the spatula to unfold or rearrange it.) Cook until bottom is firm and golden brown in spots, about 45 seconds. Transfer to a plate, and cover. (The first one will not be your finest.) Repeat with remaining batter, brushing pan lightly with butter as needed (every 2 or 3 crepes). Serve immediately.
Basic buckwheat variation: Substitute 3/4 cup buckwheat flour for 3/4 cup all-purpose flour, and add an additional 3/4 cup whole milk. (Makes 40 six-inch or 15 ten-inch crepes. )