Kaale Kaffi (as in cold coffee) is the new hype address to have coffee in Luxembourg city. Everyone knows Knopes, Golden Bean and Konrad, but Kaale Kaffi is still some sort of insider tip.
The coffee place is actually within a vintage shop in rue de la Boucherie, behind the Palais. And indeed they do serve excellent coffee! Otherwise the menu is relatively small: a few nice soft drinks and lemonades, as well as some beer and wine, everything at very reasonable prices. You can also have soup, sandwiches or cake. Everything is served with a nice smile and the whole place has just a really nice atmosphere. Cheers!