Ukishima (浮島) is a Japanese traditional sweet (wagashi).
It’s basically a two-layered steamed cake that has sweet bean paste (red or white) mixed into the batter.
It shouldn’t be confused with castella, which is a sponge cake also made in Japan.
The name means “floating island”, since the cake looks like an island floating on top of the sea.
Besides the sweet bean paste, the other ingredients are eggs (separated into egg yolk and egg white), sugar or syrup, and rice flour.
As a side note, there are different types of rice flour used in Japan.
This type is called joshinko. It isn’t the same as glutinous rice flour (mochiko), because it’s made with short grain rice instead.
Another characteristic of ukishima is that it feels moist.
Each layer can have a different color, for example a green one can be made with matcha.
This sweet isn’t as popular outside Japan.
I couldn’t find a video about it in English.
But at least I found this one with subtitles (although they could be better):
