Peel the batatas and cut into 1-inch chunks. Place them in a saucepan and cover with water. Bring to a boil and boil until they are tender but not mushy, about 10 to 15 minutes.
Drain, then place the batatas in a blender or food processor and process to a smooth puree. If the mixture is too thick (like dry mashed potatoes), add a little water.
Place the sugar, 1/2 cup water, lime and cinnamon stick in a heavy saucepan and bring to a boil over medium-high heat, stirring constantly, until the syrup reaches the soft-thread stage (230 degrees), about 15 minutes.
Reduce the heat to low, remove the lime and cinnamon stick, and add the batata puree, stirring constantly. Cook for 3 or 4 minutes, stirring, until the mixture is well blended.
Remove from the heat and stir in the beaten egg yolks. Return to low heat and cook for 1 to 2 minutes, stirring.
Stir in the Sherry and pour the mixture into a dessert dish, like a 7-inch souffle dish. Chill thoroughly before serving. Serve with whipped cream if desired.
1 1/2 pounds Caribbean sweet potatoes (these are the red-skinned, white- fleshed tubers called batatas; choose ones with light red skin -- the flesh is moister)
3/4 to 1 cup sugar, or to taste
1/2 cup water
1 quarter-size piece of lime peel
1 stick cinnamon
2 egg yolks, beaten
1/4 cup of dry Sherry (like palo or manzanilla)
Whipped cream (optional)