Wash and dry the sweet potato. Wrap up in aluminum foil and bake in the oven at 250 ℃ for 1 hour (if you steam or boil the potatoes the flavor will be different from when you bake them.)
Peel off the skins and remove any hard parts. The net weight after peeling was 370 g. You can either puree it in a food processor, or pass it through a sieve at this point if you like.
Mash the potatoes and put in a pot. Add the butter, heavy cream, granulated sugar, lemon juice, and brandy and heat over medium heat. If it's still a bit too stiff, add milk.
Use a wooden spatula to knead and mix. Once it's stiff enough to be squeezed out of a pastry bag, remove from the heat and add the egg yolk (also add vanilla essence if you like).
Pass the mixture through a sieve. If you do it at the beginning it'll be stiff and may take a lot of time. If you do it at this stage it takes a lot less time.
Put the cream in a pastry bag fitted with a star-shaped tip, and pipe it out into small baking cups or cocottes in a circular motion.
Bake in the oven at 200 ℃ until browned on top, and they're done. I made these in small paper cups.
Story Behind this Recipe
This is a recipe I've been making for several years now, and now I'm passing it on to you!
The baking time for the sweet potatoes may vary depending on the oven. If they are still too hard, lengthen the baking time. If the cream is too stiff in step 3, add some heavy cream or milk. If you want to coat the outside with egg yolk, use 1 egg yolk together mixed with a teaspoon of honey or mirin.