How to Make It
Combine sweet potatoes, rum, brown sugar, salt, and ginger in a large stockpot. Add water to cover potatoes, and bring to a boil over medium-high, stirring occasionally to dissolve sugar. Reduce heat to medium-low, and simmer until potatoes are just tender, about 12 minutes.
Preheat broiler with oven rack 6 inches from heat. Drain sweet potatoes well; transfer to a lightly greased 11- x 7-inch glass baking dish. Drizzle with melted butter. Top evenly with marshmallows and coconut. Broil until coconut and marshmallows are toasted, 1 to 2 minutes.