German chocolate cake in Brussels
They don't make cakes like that here.
I guess you mean Black Forest Gateau?
Lots of places do it, but Wittamer is pretty good:
Not sure whether the US version corresponds to anything in Germany, but the next German city is just 135 km away: Aachen. Easy to go in the morning and come back in the afternoon. Foodstuff is cheaper in Germany than in Belgium, as are postal service, gas and a few things more.
I doubt very much if you would be able to buy a (US style) German Chocolate Cake in Belgium. You best bet would probably be to go to one of the large supermarkets where they have a, "American" section, and see if you can find a Betty Crocker / Duncan Hines / Pillsbury cake mix.
The Carrefour in Waterlo has a reasonable selection of US products, and you may find one there.
In Wezembeek, there is a German shop called Gunter's something. It used to be closed but I saw it open again. Ask there. Sometimes people put ads for that kind of things in such shops. Try if I you can find something.