Heat a skillet over medium heat. Add the butter and allow to melt.
Spread the mayo on one side of both slices of bread. Then place both pieces of bread (mayo side down) in skillet on top of melted butter. Turn to medium-low heat.
Allow the edges to turn lightly golden brown and then flip. Add more butter to the pan if needed to crisp the other side. Top one of the toasted sides with cheese and allow to melt. Once almost melted, add any desired toppings and then place the other slice of bread on top.
Tip: If the cheese isn't melting fast enough, cover the pan with a lid for 1 minute.