Preheat a sandwich press. See notes in post text for approach without a sandwich press.
Spread half the butter on one side of a piece of bread. Spread the remaining butter on one side of the second piece of bread. Place both buttered sides down on a plate.
Spread the mustard on one of the unbuttered sides of bread, and set aside.
On the other unbuttered side of bread, add slice of cheese, followed by the Serrano ham. Place another slice of the cheese on top of the ham.
Close the sandwich up by taking the bread with the mustard and placing it mustard side down on the cheese.
Carefully move the sandwich and set it in the heated press. Gently press down on the sandwich maker.
The sandwich takes about 5 minutes to get the cheese melted and develop those nice griddle marks. Keep an eye on the cheese as the sandwich heats, you don’t need to cook it much past the point where the cheese has started to melt.
When the sandwich is ready, use a plastic spatula to remove the sandwich from the press. Plate the sandwich, cutting it in half if you like for presentation effects. Enjoy!