Mochi left over from the new year, gone hard on the shelf, is boiled soft again and then pounded together with steamed sweet potato until the two of them become a single substance. Nothing is measured. Rolled into pieces by hand, dusted with kinako, and eaten warm.

The name comes from a Miyazaki word meaning to pound and mix things together, and there is a second account which says it describes the sound of the work itself, bottabotta. Depending on where you happen to be standing it is nettabo, or nettakuri, or karaimo mochi, that last one because sweet potatoes were called karaimo across the old Satsuma domain.

It belongs to Miyakonojo and to the farming country around it. When households pounded mochi at the turn of the year, this was made alongside, so it is tied to that week specifically rather than to a festival.

It served as a substitute for a main meal, and also as the thing farmers ate between one job and the next. What it really is, though, is an answer to a small annual problem. Every household ends up in the middle of January with mochi that nobody wants to eat any more. Somebody here decided the solution to that involved a sweet potato.