Roasted, ground hazelnuts lends a nutty background note to this chocolate cake, which uses all-purpose and potato flour in its batter. The latter ensures a surprisingly delicate result, while making the dough easier to handle. Interstingly, the starch in potatoes also attracts and retains water, therefore helping to boost the cake;s moisture content. You'll need a ring cake to bake it, though you can substitute a bundt pan for similar results.