For mousse:

Melt butter and chocolate in a small heavy saucepan over very low heat. Stir until smooth; set aside. In small bowl of mixer, beat egg whites and salt until foamy. Gradually beat in 1/4 cup sugar until stiff peaks form. Add 1 cup heavy cream; beat until stiff and well blended; set aside.

In large bowl of mixer, beat egg yolks, 1/4 cup sugar and the vanilla or rum until well blended. Fold in chocolate mixture; then gently fold in egg-white mixture just to blend. Spoon into 4 dessert cups. Chill several hours. Serve topped with mocha whipped cream.

For mocha whipped cream:

In small chilled bowl of mixer, whip with chilled beaters 1/2 cup heavy cream, 1 teaspoon instant coffee, 1 tablespoon confectioner's sugar and 1/4 teaspoon vanilla until soft peaks form.