I don’t see how the mayonnaise is going to change anything. Mayonnaise, first of all, is much easier to spread than butter. When spreading over softer bread, this will help keep it from crumbling and tearing. Second, unlike butter, mayo does not burn quickly due to its higher smoke point. Finally, the mayonnaise’s egg and oil components set the bread to a beautiful brown and impart a creamy taste to the crust.
Your family is going to love your grilled cheese sandwiches when you learn this one little trick!