Mix the eggs, milk, flour, sugar and salt with a whisk in a measuring cup (must be very thin, almost liquid). Cover the measuring beaker with cling film and let it rest for 1 hour in the refrigerator.
Meanwhile, grate the cheese and cut the ham into cubes. Heat a pan (Teflon), spread thinly with butter, preferably with a brush. Put half a ladle of batter per crepe into the pan and distribute the batter by moving the pan. The crepe is turned over when it blisters and the surface is no longer moist, then fry for 30-40 seconds from the other side, while placing the ham and cheese on it so that it melts slightly. Fold up and serve.