Boil or microwave the sweet potato until soft.

Mash the cooked sweet potato thoroughly.

In a bowl, combine the mashed sweet potato and katakuriko.

Gradually add water while mixing, until the mixture reaches your desired stiffness (approximately 150 ml).

Moisten your hands with water to prevent sticking.

Shape the mixture into small, flat patties or desired shapes.

Heat a frying pan with a small amount of oil over medium heat.

Place the shaped patties in the hot pan and cook until golden brown on both sides.

In a separate saucepan, combine soy sauce, sugar, mirin, and water.

Bring the mixture to a boil.

Mix katakuriko slurry with water until homogenous.

Add katakuriko slurry to the mixture to thicken the sauce.

Simmer until the sauce reaches your desired consistency.

Remove the cooked oyaki from the pan and serve with the thickened sauce.