Preheat oven to 425°. Prick sweet potatoes with a fork and put in a baking pan. Bake in middle of the oven until tender (approx. 1 hour). When cool enough to handle, peel potatoes and transfer half to a food processor. Add bourbon and 6 tablespoons of butter and puree for 30 seconds. Transfer puree to a large bowl.
Puree remaining potatoes in food processor until completely smooth and transfer to bowl. Stir puree until combined well and season with salt and pepper. Transfer puree to a 2-quart gratin dish or other shallow baking dish. Note: Puree may be made 2 days ahead and chilled, covered. Bring puree to room temperature before proceeding.
Reduce oven temperature to 325°. In shallow baking pan spread pecans in one layer and bake in middle of oven until fragrant, about 10 minutes. Toss hot pecans with remaining 2 tablespoons of butter and salt. Pecans may be made two days ahead and kept in an airtight container at room temperature.
Arrange pecans on top of puree and sprinkle with brown sugar. Bake puree in upper third of oven until heated through and pecans are slightly browned, about 20 minutes. Makes 8 servings.