Reheat oven to 350 degrees F. Bake potatoes until soft throughout, about 1 hour. Cool, then peel and cut into chunks. Puree in a food processor--do not over mix--and reserve. Reduce oven to 325 degrees F.
Prepare caramel and sauce following the same method as for Creme Caramel (recipe follows). Line a 3-quart Pyrex loaf pan with caramel and set aside on a level surface. Chill sauce.
Combine custard ingredients in a large bowl and whisk in the sweet potato puree to combine. Pour through a strainer into lined loaf pan.
Place inside a larger roasting pan. Pour in boiling water until it rises halfway up the sides of the loaf pan. Bake about 1 hour and 50 minutes. The center should feel firm when pressed.
Set aside to cool about 1 1/2 hours. Cover with plastic wrap touching top. Refrigerate overnight or as long as 4 days.
Run a knife along inside edge 2 or 3 times to loosen. When you press center, sides should pull away. Cover with a platter and quickly invert. Drain excess caramel into a bowl and strain into reserved cold sauce. Serve slices of flan on dessert plates. Top with chilled caramel sauce.
c.1997, M.S. Milliken & S. Feniger, all rights reserved