Butter one side of each bread slice. Spread mayonnaise and mustard on unbuttered sides. Divide and top two of the unbuttered sides with the turkey and cheese. Top with the other slices of bread, buttered side up. Heat a non-stick grill pan or skillet over medium heat. Set in the sandwiches and cook 2-3 minutes on each side, or until cheese is melted. Slice sandwich in half and serve.