Pour brandy over raisins in a small bowl, cover, and allow to stand for 2 hours.
Preheat oven to 350 degrees F (175 degrees C). Butter a 2-quart baking dish with the softened butter.
Place sweet potatoes on a baking sheet, and bake in preheated oven until tender, about 30 minutes. Remove and allow to cool until cool enough to handle, then peel the potatoes and slice them 1/2 inch thick. Arrange the potato slices in the prepared baking dish, and sprinkle with sugar and brandy-soaked raisins.
Return the sweet potatoes to the oven, and bake 30 to 40 minutes until hot and bubbly.