Cover the boniato chunks with cold water in a pot, add the salt, and bring to a boil. Simmer 20 to 25 minutes, until a fork meets no resistance.

Drain thoroughly and let the chunks steam dry in the hot pot for a minute. Mash, then push through a fine mesh sieve or process until completely smooth with no lumps.

In a wide heavy saucepan, combine the sugar, water, cinnamon stick, and lime peel. Bring to a boil, then simmer 10 minutes until it forms a light syrup that thinly coats a spoon. Remove and discard the cinnamon stick and lime peel.

Stir the boniato puree into the syrup until smooth. Cook over medium-low heat for 15 to 20 minutes, stirring often, until a spoon dragged across the bottom leaves a track that holds for a second.

Take the pan off the heat and let it stand 2 minutes. Beat the egg yolks in a small bowl. Ladle ½ cup of the hot puree into the yolks in a thin stream while whisking constantly, then repeat with another ½ cup.

Pour the warmed yolk mixture back into the pan while whisking. Return to LOW heat and stir constantly with a wooden spoon for 4 to 6 minutes, until it thickens and the spoon leaves a clean parted path across the pan bottom. Do not let it boil.

Off the heat, stir in the vanilla or sherry. Divide among 8 small cups or ramekins, about ⅓ cup each, and smooth the tops.

Refrigerate at least 3 hours or overnight. Dust with ground cinnamon just before serving.