Greek walnut and coffee cake

I ate a version of this on the Greek island of Ithaca, but I bought something very similar in a shop selling baklava in London. It took me quite a while to find a taste that was similar using my home ingredients (I am sure they do not use butter in Greece) and here is my attempt. This is a grown-up cake. It can be eaten as an afternoon tea cake, but I also like to serve it as a pudding in the summer with a big dollop of Greek yoghurt alongside.