Salted Butter Caramel-Chocolate Mousse

With butter-and-cream richness, bittersweet notes from the chocolate and caramel, and sea salt to cut through the sugar, this simple six-ingredient dessert from “My Paris Kitchen” by David Lebovitz is far greater than the sum of its parts. You can whip the egg whites by hand using a whisk, or use a handheld or stand mixer. Whatever method, make sure the bowl and whisk are perfectly clean and free of any residual oil, which will prevent the egg whites from achieving maximum loft. (Note that the eggs here are not cooked.) Fleur de sel is a hand-harvested, somewhat coarse-grained sea salt from France. Just about any variety of finishing sea salt can be substituted, but don't use very coarse salt (the type meant for grinding). The salt particles in the mousse don't fully dissolve; the little bursts of salinity are what makes this dessert so unique and delicious.