Put the dark chocolate, butter and coffee in a small saucepan over a low heat. Stir until melted and smooth. Stir in the egg yolks and salt. Set aside.
Using an electric mixer, beat the egg whites on a high speed until soft peaks form. Add the sugar, slowly, and beat until stiff peaks form.
Fold a little of the egg whites into the chocolate mixture until it is smooth. Then fold all the chocolate mixture into the egg whites. Fold in the cream.
Pour the mousse into 6 glasses. Refrigerate until set, preferably overnight. Serves 6.
Recipe by Butter Baking at https://www.butterbaking.com/2012/09/23/chocolate-espresso-mousse/