Egg, sugar, flour and starch syrup are poured into a wooden frame and baked. Coarse sugar settles at the bottom during baking and stays there, so the first bite has a faint grit in it that the rest of the cake does not. Look at a cut face and the crumb is unusually tight and even. No raising agent is used at any stage.

The name is generally traced to a Portuguese phrase meaning bread of Castile. There is also a story in which a Japanese asked a Portuguese what the cake was called and was told it was a cake of the kingdom of Castile, and that answer became the name. Neither version can be proved and both are still repeated.

It arrived around 1571, at a Nagasaki harbour that had only just opened to trade. For most of the Edo period it stayed expensive, a thing sent as a gift to magistrates and to lords. What happened afterwards is the interesting part. Local confectioners kept adding egg, introduced starch syrup, and baked it moist rather than airy, deliberately closing the crumb instead of lifting it. Everything European baking was trying to achieve, they went the other way on.

It came from abroad and it is now counted, without argument, as a Japanese confection. Four hundred years is apparently how long that takes.