According to the write up on Just One Cookbook, it is a form of ganache, specifically called Pave de Geneva, created in the 1930s.

Just One Cookbook is great. I’ve used their recipes for years and have always gotten satisfactory or better results (any shortcomings are my own, not the recipes’). The first recipe I attempted from there was for anmitsu to try to impress my now-wife. I suppose it worked, but talk about false advertising…

I also reference it every time I want to make dashi from scratch, or get the timing / technique for onsen tamago, or any number of other fun dishes.