In a glass bowl, in bain marie, melt the chocolate, stirring a few times. Let it cool. In a stainless steel bowl, beat the cream until it becomes soft, then fold in the chocolate, chestnuts and a pinch of salt. In a large stainless steel bowl, beat the egg whites until very soft. 
While beating, gradually add the sugar and beat until firm and glossy, 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 bowls. Cover and refrigerate until firm, at least 2 hours. Serve each mousse with a couple of frozen raspberries and 1 sprig of mint.