Melt the butter in a saucepan over medium high heat and sweat the shallots and garlic. Add the rum, bring to a boil and reduce to 1/3 cup. Add the stock, bring to a boil and reduce the heat. Whisk in the remaining ingredients and simmer until reduced to 2 cups.
Preheat the grill. Season the steaks on both sides with salt and pepper to taste. Grill to desired doneness. Drizzle with the sauce.