This chocolate mousse cake ticks a lot of boxes—big, grand, ideal for the holidays, and just enough of a challenge to ensure you feel accomplished. The one-bowl chocolate cake layer is dense and deeply fudgy; the mousse is airy and light; and the thin slip of ganache on top brings it all together. No matter what kind of year you’ve had, this dessert is the way to cap it all off. Making the mousse alone will teach you three foundational pastry techniques, and they’re all hidden in plain sight.