This is where you can buy it Suteria - Confiserie a long established café/cake shop in Solothurn, where I live (well a kilometer down the road from my village). I only found the web site in German, but there are photos to show everything they make. It is the only place in Switzerland where you can buy it and you can even send them as presents by post, but express post, otherwise they might perish on the way. Made from fresh ingredients, but from what no-one really knows.
The original owner of the shop, Mr. Fluckiger, sold the recipe to Mr. Suter for a very high price and the ingredients and way to make the cake are top secret. You can find various recipes in Internet or cookery books, but they are not original. Somewhere the recipe is probably secure in a bank safe.
I think it is based on hazelnuts. Although I am not a great fan of hazelnuts, they are applied in such a way that the cake is just plain delicious. They come in all sizes, as you can see on the photo and naturally all have the same design. They are covered with a firm glazing. Inside there are two halves, on the spongy side, and in the middle a very rich creamy filling containing ground hazelnuts, at least I think so. Under the glazing at the top, bottom and sides, there is also a thin layer of the cream. They are not cheap, the largest about 30 swiss francs, but they are good.
I happened to be in town this afternoon with my camera (what a coincidence) and took this photo in the window of the café.