Here's a marvelous cake for the holiday season, conjuring images of chestnuts roasting on an open fire. Chestnut flour can be found at gourmet food shops and some natural food stores and Italian markets (or online). Orange and clementine zest impart even more wintry tastes, while honey, brown sugar, and yogurt, boost the moistness quotient. The chocolate ganache that's poured over the top and sets to a glossy sheen is a festive finale.