I do remember some birthday parties of a late relative with an abundance of tasty cakes, some of them buttercream cakes. Now, I don't know the recipes...

I do.

I remember having eaten this, not many but several times, in places called "Konditorei". Like a coffee shop, but with additional bakery/pastries. Was really good actually, but there were many other options which were at least as good. "Schwarzwälder Kirsch" f.i.

So by no one I assume you mean the "fortgeschrittene Fertigfutterfresser" who prefer other convenient stuff, and/or have no Konditorei near them?

Guessed so...

Maybe it is a regional thing, but I (German) can't remember to have ever seen this and as a child I was very interested in the sweeter goods offered by local bakeries.

Buttercreme is an old-fashioned frosting you'd find more towards the middle and north of Germany.

I've had one last week, in Germany.

We had this oftentimes when I was young. You would have them e.g. on your grandma's birthday or other special occasions. I think it was more an old people's thing, but they still sell them in bakeries. This is in the west of the country..