Pre-heat cast iron skillet over medium high heat.
Butter one side of each piece of bread and apply a thin layer of mayonnaise on the other.
Assemble the grilled cheese by placing one slice of bread, butter side down, on the hot skillet. Place one slice of cheddar cheese on the bread, top with optional toppings, and complete with slice of other type of cheese and slide of bread with butter side up. Repeat for remaining sandwich.
Press the top of each sandwich down lightly with a spatula and let cook until bread is golden brown and cheese is melted, about 3 minutes per side.
Remove from the skillet, slice diagonally and serve immediately.