Preheat 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.