This rich, yet airy five-ingredient dessert requires no cooking and comes together in minutes. Just make sure that the mascarpone is softened to cool room temperature so it combines easily with the egg yolks. A hand mixer makes easy work of whipping the egg whites, but you also could use a whisk and little elbow grease. Serve the mousse as soon as it's made, or cover and refrigerate for up to 45 minutes. A dusting of cocoa adds visual appeal as well as a hint of chocolate flavor; fresh berries or crisp cookies also are excellent. Note that the eggs here are not cooked. If rum isn't your thing, substitute 2 teaspoons vanilla extract, or simply leave it out.