Divide half the cheese slices onto 4 bread slices. Top with sauerkraut, dressing, Serrano ham and remaining cheese. Cap with remaining bread slices. Spread butter on outsides of sandwiches.
Heat skillet over medium-low heat. Pan-fry sandwiches 5 to 7 min. turning once, until the bread is toasted and cheese is melted. Cut in half and serve with pickles, if desired.
Alternatively, grill sandwiches in panini press.