Preheat oven to 375 degrees. Prick sweet potatoes all over with a fork. Bake on a baking sheet until soft, about 1 hour. Let cool slightly. Peel potatoes, transfer to a bowl, and mash with a potato masher.
Bring coconut milk and curry paste to a simmer in a saucepan. Cook for 5 minutes. Add potatoes along with 3 tablespoons maple syrup, 2 tablespoons butter, and the salt, and stir.
Preheat broiler. Butter six 1-cup gratin dishes or one 6-cup baking dish. Spoon mixture into dishes, and smooth tops. Drizzle with remaining maple syrup. Dot with remaining butter, cut into small pieces. Broil until tops are sizzling and browned, 3 to 4 minutes.