What is that solid layer of chocolate on top of some chocolate cakes?

I already know this is a badly worded question. I can’t figure out a better way to say it though. I don’t think I’ve seen such a cake in America, but in some other countries, the chocolate cakes have a solid layer of chocolate on top of them. Almost like a thin chocolate bar. I don’t know how else to describe it. I don’t think it’s frosting, because it’s too thick (and solid) to be that. But I could be wrong.

Any idea what this solid layer is? And how I can make it and add it to my own cakes? Sorry for the poor description. Thanks for all help in advance!