Melt the chocolate and butter in a metal bowl placed over a pot or barely simmering water, stirring gently until melted. Remove from the heat and allow to cool while preparing the other mousse components.
Whip the cream to soft peaks and stir in the espresso and brandy. Chill until ready to use.
In a clean bowl, whip the egg whites with the salt until foamy, then gradually add the sugar while continuing to whip the whites until they hold a soft peak when the beaters are lifted.
By now the chocolate should be cooled to just above room temperature. Fold the whipped cream quickly into the chocolate in 2 additions, then fold in the whites all at once (the mousse will be soft). Pour this into desired serving dishes and chill until set, about 3 hours. Top with chocolate shavings before serving.