Preheat the panini maker on the grill-panini setting to Medium or let nonstick pan heat up on low heat on the stove.
Spread butter to the very edges on one side of each slice of bread. Put the cheese in the middle of the two slices of bread with the butter on the outside.
Put the sandwich into the panini maker and let cook a few minutes so the bread defrosts. Turn the panini maker onto High heat and let the grilled cheese cook until the bread looks crispy and brown and the cheese is oozing out the sides.
If you're using a nonstick pan, put the sandwich on the pan and turn up the heat to medium. Keep an eye on it and flip to the other side once the bread turns crispy and brown.