Preheat your oven to 400 degrees Fahrenheit.
Spread 3 tablespoons of softened unsalted butter over one side of half your sandwich bread slices.
Place the buttered slices of bread butter side down on a rimmed baking sheet.
Layer each slice with one slice of medium cheddar cheese and one slice of mozzarella cheese.
Top with another slice of sandwich bread, butter side up.
Spread remaining butter over the top of the new slices of bread.
Bake for about 10 minutes, flipping halfway through.
Once golden brown, remove and let cool for a minute before slicing.
Serve warm and enjoy!