Peel the sweet potato and slice into rounds. Put them in a pot with enough water to cover, and boil. Once a skewer goes through the sweet potatoes easily, drain well.

Put the sweet potato in a bowl, mash with a rolling pin, and pass through a strainer. The texture will become smooth by straining the mashed potato (straining is optional).

Add the ★ butter, sugar, egg yolk, and heavy cream to the mashed potato, in that order, mixing well after each addition.

Cut off a corner of the bottom of a plastic bag, and fill with the Step 3 mixture. Pipe the mixture into small baking cups and bake in a toaster oven, making sure not to brown them too much.