2Prick the sweet potatoes all over with a fork. Place on a baking sheet and bake 40 to 60 minutes, or until very soft. (To cook the potatoes in a microwave oven, prick, wrap in paper towels, and cook on high 5 to 8 minutes, or until soft, turning once. Remove paper towel, wrap potatoes in foil, and let stand 3 to 5 minutes. Proceed as directed.) Cool until easy to handle, then remove and discard the peel. Puree the flesh in a food processor or blender, then press through a sieve. Measure out 1 cup puree (use any excess for making sweet potato pasta, or to add to soups or stews) and set aside.
3In a small saucepan, scald milk and set aside.
4In a bowl, beat egg yolks well. Beat in sugar, salt and spices. Add hot milk slowly, whisking constantly. Pour mixture into the top of a double boiler and cook over barely simmering water, stirring constantly, 5 to 8 minutes or until mixture coats the back of a spoon.
5Turn off the heat, whisk in the potato puree. Cool to room temperature; stir in cream and vanilla. Chill in the refrigerator for at least 1 hour. Freeze in an ice-cream maker according to manufacturer's directions. Fold in nuts, if desired.