Melt the chocolate and the butter in Bain-Marie. When the chocolate starts to melt add coffee powder, cocoa powder and vanilla extract. Meanwhile, grind the biscuits and the nuts in a food possessor into medium size crumbs. Save some crumbs for decoration. When the chocolate and the butter turn to a smooth runny mixture, turn off the heat and let it stand for a few minutes. Than add the crumbs to the mixture and stir.
As soon as possible (don't let the mixture to cool down to much) start mixing it with hands. If you feel that there is not enough liquid and its too dry, add some milk. If the mixture is too runny and you cant form it into anything, add more crumbs.
Form the mixture into small compressed balls and roll them in more crumbs, cocoa powder or coconut. You can also melt additional 100 grams of chocolate and dip the balls in it. When done, put the balls on a parchment paper covered plate or tray and put it in the fridge. Let the chocolate balls to set for hours. Serve cold.