The tanginess of the crème fraîche is soothed by the rich and creamy chocolate mousse, which is in turn lightened by the crème fraîche. This recipe uses uncooked egg whites and yolks. Small children or people with compromised immune systems should try another recipe.
In a saucepan over very low heat, melt the chocolate and the butter together. Just before the chocolate finishes melting, remove it from the heat and beat it with a whisk until smooth. Transfer the chocolate mixture to a bowl and beat in the egg yolks with the whisk. Refrigerate. Beat the egg whites with 2 tablespoons of the sugar until the whites hold stiff peaks but are not dry. Set aside. Beat the cream with the remaining 2 tablespoons sugar and the vanilla until it holds soft peaks.
Stir a couple of spoonfuls of the whipped whites into the chocolate mixture to lighten it, then fold in the remaining whites thoroughly but gently. Fold in the cream and refrigerate the mousse until chilled. If you are in a hurry, divide the mousse mixture among six cups; it will chill much faster. Serve the mousse within a day or two of making, under a generous scoop of crème fraîche.