Preheat oven to 400. Prick sweet potatoes. Place on a baking sheet and cook until soft, about 40 minutes. Let cool. Peel sweet potatoes, cut into chunks, and set aside. Melt 2 tbsp. of the butter in an 8" skillet over medium heat. Reduce heat to medium low, and add onion; cook, stirring occasionally, until caramelized, 6 to 8 minutes. Add vinegar; cook 2 minutes. Stir in ginger, mustard, cayenne, salt, and pepper. Cook over low heat 2 to 3 minutes. Place sweet potatoes in a blender/food processor; add onion mixture; process until smooth. Transfer mixture to a heavy saucepan. Over low heat, cook 1 hour, stirring often, until mixture takes on a deep, rich color. Cool and then beat in the remaining 4 tbsp. of butter. Makes 2 cups. This butter is delicious spread on warm biscuits.