Preheat the oven to 190°C, gas mark 5. Grease an ovenproof dish.
In a bowl, beat together the butter, mustard, garlic and parsley.
Arrange the potato buns in the dish. Make a cut in each one without slicing all the way down. Brush in and out with the butter, scatter the cheddar inside and press together. Bake for 8-10 minutes, or until the buns are golden and cheese has melted.