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 1½ minutes until the underside is golden brown (when the top of the crepe is opaque, you can look at the underside by lifting the edges gently with a flat knife). Toss or turn the crepe and, if necessary, straighten it out with a knife.
Turn down the heat in the pancake pan and, if you want to add a ham, lay a slice of cooked ham on top of the crepe. Transfer the cooked egg on top of the ham. Cook for about 2-3 minutes until the underside of the crepe is cooked then, from four points, fold the edges of the pancake towards the middle to create a square envelope, leaving the yolk of the egg exposed. Serve and eat immediately.