Put the flour in a bowl and stir in a little milk to moisten. Stir in the egg, the salt and the remaining milk until the batter is smooth. Let stand at least 20 minutes and stir before using.
Preheat the oven to 70°C (approximately 160°F). Heat some butter in a pan, pour a ladleful of batter into the hot pan, and gently swirl the pan until the batter covers the bottom in a thin layer. Cook over medium heat until lightly browned on the underside. Flip the crepe over and cook until the underside is lightly browned. Repeat with the remaining batter.
Transfer to a plate, keep warm in the oven while you prepare the remaining crepes (you'll have about 8 crepes).