Mix the flour and salt together in a mixing bowl. Make a well in the centre and place the egg and half the milk into the well. Whisk, gradually incorporating the dry flour into the wet mixture, then beat thoroughly until smooth. Whisk in the remaining milk and the melted butter, a little at a time. The consistency should be similar to single cream (half-and-half). You can add a little extra milk if necessary.
Lightly grease a flat-bottomed frying pan (skillet) with butter then place it over a moderate heat until it is hot enough for frying (a drop of batter should brown on the underside almost immediately). Pour in just enough batter to cover the base of the pan, tilting the pan to spread the batter evenly. Cook for about 2 minutes until the underside is golden brown (when the top of the crêpe is opaque, you can look at the underside by lifting the edges gently with a flat knife). Toss or turn the crêpe and, if necessary, straighten it out with a knife.
Cook on the other side for between 1 and 2 minutes, till golden. After the crêpe has finished cooking it resembles a really thin version of a sweet breakfast pancake