If the macaroni and cheese is cold (leftovers), heat the mac and cheese by placing it in a non stick pan over medium heat for 2 to 3 minutes.
Butter one side of each piece of your bread, then in a separate pan place the bread butter face-down over medium heat. Place a slice of cheddar on each piece and allow the cheese to melt while grilling the bread to a golden brown.
Shape the mac and cheese into a square that will fit onto your bread of choice, then top the mac and cheese with the remaining slice of cheddar and allow the cheddar to melt. Sprinkle the melted cheese with the crushed red pepper.
Transfer the mac and cheese square to one of the pieces of bread, then top with the other (grilled side up). If needed, continue to grill until the sandwich is golden brown on both sides.