1Preheat skillet over medium heat. Butter each slice of bread on 1 side only. Spread mayonnaise on the un-buttered sides. Place 1 slice of bread, buttered side down, in the skillet. Layer corned beef and cheese slices on bread. Cover with remaining slice of bread, buttered side up. Cook for 5 minutes on each side, or until toast is golden brown, and cheese is melted.