Grease the bottom and sides of a 1.25 quart casserole dish with the butter.
Spoon the frosting into the dish and spread it evenly so it covers the whole bottom.
Cover the entire surface of the frosting with the marshmallows.
Place the dish in your oven's middle shelf and bake for about 13 minutes until the marshmallows are light golden brown. Watch carefully to make sure they don't burn.
Remove the dish from the oven and let the dip cool for about 5 minutes. Serve while still quite warm with graham crackers, vanilla wafers, or your favorite cookies. You can reheat the dip in a microwave at 80% power for 30-second intervals until it's as warm as you want.
This recipe serves approximately 8 people and makes a particularly awesome slumber party treat.