Melt the Butter - In a small bowl, add the butter and use 15-second intervals on high power in the microwave, 60 to 75 seconds. Evenly brush the melted butter on one side of each slice of bread.
Add the Cheese - Add 1 to 2 slices of cheese between the unbuttered sides of the bread to make a sandwich.
Cook First Side - Heat a large cast iron skillet or nonstick pan over medium-low heat, adjusting to low if needed. Once hot, add the sandwiches to the pan. Lightly press down with a spatula, then do not move the sandwich. Cook until the bread is crisp and golden brown, about 3 to 5 minutes.
Flip Sandwich - Flip and cook until golden brown, and the cheese is melted, about 3 to 5 minutes.
Cut & Serve - Slice diagonally and serve immediately while still warm.
Notes
Bread Options: White, whole wheat, squaw, sourdough, or brioche. Cook sweeter pieces of bread on low heat as the sugar brown quicker.
Cheese Options: Any type of firm, low melting point cheese can be used, like cheddar, mozzarella, provolone, swiss, gouda, Monterey Jack, or pepper jack.
Using Shredded Cheese: 1 cup of shredded cheese can be used instead of slices. Use about ½ cup per sandwich.
Using Extra Butter: The sandwiches can be fried in some extra butter, however, cook on low heat, so the milk solids don't burn. Wipe the pan clean and add fresh butter if making more than two sandwiches.
Butter Alternative: Mayonnaise or olive oil can be used to coat the bread instead of butter.