Heat a large skillet or griddle over medium heat. Butter the outsides of the banana bread, and place one slice butter-side down in the skillet. Immediately top the slice with some cheese, a bit of chocolate, and a little more cheese. I like to always have cheese on both sides to act like “glue” – it helps hold the sandwich together when flipping. Cook until both sides are golden and the cheese and chocolate is barely melted, about 4 minutes per side. If the cheese is melting slowly, I like to reduce the heat to medium-low and cover the skillet – just make sure to watch the sandwich because the sides cook more quickly.