Have skillet already heated. Slice each cheese in 1/2 and have ready.
Butter 1 side of each slice of bread and place 1 slice, buttered side down, on very warm skillet. Place 1/2 slice cheddar and 1/2 slice mozzarella on the unbuttered side of bread in the skillet. Place the other slice of bread, butter side up on top of the cheeses.
Brown to taste on one side and flip to brown on the other side. Take from skillet and slice diagonally through the stack.