Candied Sweet-Potato Purée

Preheat oven to 400 °F. and line a baking sheet with foil. Prick sweet potatoes in several places with a fork and bake on baking sheet in middle of oven until very tender, 1 to 1 1/4 hours. Finely grate enough zest from orange to measure 1 teaspoon and squeeze enough juice from orange to measure 1/2 cup. In a small saucepan melt 4 tablespoons butter over moderate heat and stir in zest, orange juice, sugar, bourbon, nutmeg, and salt, stirring until sugar is dissolved. When potatoes are just cool enough to handle, peel and put flesh in a food processor. Add butter mixture and pur e just until smooth. Sweet-potato purée may be made up to this point 2 days ahead and chilled in a 1 1/2-quart shallow baking dish, covered. Reheat purée, covered, in middle of 350 °F. oven. Transfer purée to a serving dish and dot with remaining 2 tablespoons butter, letting it melt to form pools.