Preparation
Cook boniato in boiling lightly salted water for about 20 minutes.
Drain the boniato and purée in a blender. Set aside.
Add water, sugar, cinnamon stick and lime peel to a large saucepan.
Cook over a medium heat stirring constantly until syrup starts to thicken (approx 15-20 minutes).
Reduce heat and remove cinnamon and peel.
Add the boniato purée. Still constantly stirring, continue for 5-10 minutes until the mixture is smooth, then remove from heat.
Beat the egg yolks with butter and a pinch of salt until smooth.
Combine both mixtures, then return to cook for a few minutes over a low heat.
Remove from heat and add the vanilla and sherry.
Leave to cool then transfer to serving dishes and refrigerate.