Wheat dough wrapped around sweet bean paste and steamed. Baking soda goes into the dough, and that is what lifts it in the steamer. The result is sweet with a faint bitterness sitting underneath.

The name comes from the raising agent. Baking soda contains sodium carbonate, tansan, and the bun is named after it. It is also called fukashi manju, steamed manju, or inaka manju, country manju, depending on the district you are in.

Gunma has volcanic soil that drains well, and long hours of winter daylight, and wheat does well under those conditions. That is why so many of the prefecture's regional dishes turn out to be made of flour rather than of rice.

In a time when sweets were genuinely rare, one made with expensive sugar was not an everyday thing. It was brought out to entertain guests at seasonal observances like the fifteenth-night moon viewing, eaten as a reward when a stretch of farm work was finished, and offered to the ancestors. It is made across the prefecture, particularly in Tatebayashi and Takayama village. It was also carried out to the fields and eaten between stretches of work. A bun named not for what is in it, or for where it is from, but for the powder that makes it rise.