Cut the sweet potato into 2 cm thick round slices. Soak in water for 5 minutes to remove the bitterness. Steam in a steamer until it becomes soft. Alternatively, you can microwave or boil it.
Peel the skin while it's still hot, and strain with a strainer. When you are not squeezing out the batter from an icing bag into the cups, you can mash in a food processor or in a bowl.
Transfer the strained sweet potato into a pot. Add the white sugar, and mix with a wooden spatula. Add the butter, egg yolk, and a pinch of salt, then mix.
Pour in the milk and heavy cream, and knead the batter. You can substitute heavy cream with milk. The batter consistency differs depending on the sweetness and the water content of the sweet potato. If it's too dry, add more heavy cream.
Knead the batter over the heat, and make it smooth. When the bottom of the pot starts to sizzle, and the batter comes off the pot easily, turn off the heat. Preheat the oven for 200℃.
Transfer the batter into the icing bag, and squeeze it out into cups. You can also put spoonful of batter in them.
Sprinkle with cinnamon powder, almonds, and black sesame seeds on top, and bake in the oven.
Bake for 10 minutes at 200℃ until browned.