Wolfgang Leschanz, an extraordinary baker who has worked at both Sacher and Demel, spent an afternoon with me demonstrating his specialties. This two-tone chocolate cake is deceptively easy for all its richness. Look at the ingredients: both the filling and the cream topping are practically identical in ingredients and proportions, but they are manipulated to give amazingly different results. The first mixture is baked and cools into a thin, fudgy cake that falls in the center. It’s supposed to do that—all the better for spreading with the chocolate mousse filling.