A bûche de Noël—a cake decorated like a yule log—is a traditional dessert served around Christmastime in France, Belgium, Canada and some other French-speaking countries. Our recipe, which is just as tasty served cold as it is at room temperature, is lighter than air. The chocolate brown-butter génoise cake made with whole-grain flour is easy to roll up and it's covered with a billowy espresso-spiked 7-minute frosting. We've substituted agave nectar for white sugar in the frosting. It's pretty garnished with meringue mushrooms and sugared rosemary.