Cook Canadian bacon in large frying pan over medium-low heat for 3 minutes on each side until hot. Leave in pan.
Combine marmalade and mustard and pour over bacon in pan. Simmer 10 minutes, turning bacon often to coat in sauce.
Place bread slices on ungreased baking sheet. Spread 1/2 tablespoon butter on top of each slice. Top with bacon slices. Spoon 1 teaspoon sauce over each slice of bacon, then top each with 2 slices of cheese.
Broil sandwiches 3 inches from heat for 3 minutes, or until cheese melts.