Whip the egg with a touch of salt. Melt the butter in a 14 cm(6 inch) non-stick frying pan over medium heat until it is fizzling. Pour in the egg and swirl it around the pan to cover the bottom evenly. Lower the heat a little and fry the egg, making some holes when it starts to set to allow the uncooked egg to run through. Shake the pan to dislodge the omelette, then scatter the ham and cheese over the top. Roll it up gently, slide it out onto a plate and serve immediately.