The name of this dessert is potentially misleading. It’s not exactly a cake, nor is it a bowl of fluffy mousse. But imagine the two concepts combined in a sublime chocolate confection that literally melts in your mouth. It’s flourless, too, which is wonderful for gluten-free diets, Passover, and frankly, for those chocolate fanatics who prefer their chocolate straight up—or chilled in a springform pan, as it were. The beauty of this dessert is that for all of its chocolate intensity, it is creamy and smooth without the density and darkness of a traditional flourless chocolate cake. This is thanks to the mousse component with its addition of whipped cream and egg whites. It’s also a convenient do-ahead dessert, since it should be prepared one day in advance of serving. You can prepare it in a loaf pan or a terrine; however, I like to use a 6-inch springform, which …