Unsweetened whipped cream and crushed chocolate-covered espresso beans, for serving
In a glass bowl, microwave the chocolates at 20-second intervals, stirring a few times, until melted. Let cool to warm.
In a stainless steel bowl, beat the cream until soft peaks form, then fold in the chocolate, chestnuts and a pinch of salt.
In a large stainless steel or copper bowl, beat the egg whites to very soft peaks. While beating, gradually add the sugar and beat until firm, glossy peaks form, about 2 minutes. Stir one-third of the beaten whites into the chocolate mixture, then fold in the remaining whites.
Spoon the mousse into ten 2/3-cup ramekins. Cover and refrigerate until firm, at least 2 hours. Serve each mousse with a dollop of whipped cream and crushed chocolate-covered espresso beans.