Place the parboiled sweet potato, double cream, cardamom pods, nutmeg and ground ginger into a small saucepan and bring to the boil. Reduce the heat until the mixture is simmering, then simmer until the sweet potato is tender. Remove the cardamom pods and blend the mixture with a stick blender until smooth.
Place one tablespoon of sugar and all of the egg yolks into a small bowl and whisk until pale and frothy. Pour over the hot blended sweet potato mixture and whisk until thickened, then pour into a ramekin dish.
To serve, sprinkle with the remaining sugar and use a chefs' mini-blow torch to caramelise the sugar.