Assemble the sandwich. Lay out the bread slices. If using, spread the mustard on the inside of one slice.
Place the cheese on top and close the sandwich with the second slice of bread.
Butter the bread. Evenly spread 1 tablespoon (14g / 0.49 oz) of the softened butter over the entire top surface of the sandwich.
Heat the pan.
Place a non-stick skillet over medium-low heat. Carefully place the sandwich in the pan, butter-side down.
Cook the first side.
While it cooks, spread the remaining 1 tablespoon (14g / 0.49 oz) of butter on the new top side.
Grill for 3-4 minutes, until the bottom is golden brown and crisp.
Flip and finish.
Flip the sandwich and cook for another 3-4 minutes, until the second side is also golden brown and the cheese is fully melted and gooey.
Serve immediately.
Remove the sandwich from the pan, slice in half, and enjoy the cheese pull!