Preheat the panini to medium-high heat. (Alternately, heat a large non-stick pan over medium-high heat.)
Arrange half the cheese on two pieces of bread, then top each slice with half the chocolate.
Top each stacked slice of bread with another slice of bread to form sandwiches and brush the tops with melted butter.
Place one of the sandwiches butter-side down onto the panini press. Butter the top piece of bread, then close the panini and allow the sandwich to cook for 2 minutes, or until the bread is golden brown and toasted. (Alternately, place the sandwich into the pan and cover it with a lid. Once the first side is toasted, butter the top, flip the sandwich and toast the second side.)
Remove the toasted sandwich from the panini or the pan, slice it in half and serve immediately.