If you use unsalted butter, add a pinch of salt to the pan to add more flavor.
Good melting cheeses: cheddar, Red Leicester, Gruyere, grated Parmesan, Fontina, Emmental, Muenster, Gouda Poor melting cheese (pair with one of the above): mozzarella, feta, provolone
Oven (normal bake): Preheat to 250ºC (480ºF). Oven (overhead broil): Do not preheat. Move the oven rack to the lowest position to reduce heat. Toaster oven: Do not preheat. If there’s a temperature control, set it to 250ºC (480ºF).
Grated Parmesan works well too.
This may only take 1–2 minutes if using a powerful broiler.
For more color, save the paprika and sprinkle it on the plate after serving.