Melt chocolate any of the following three ways: in medium bowl set over large saucepan of barely simmering water; in uncovered Pyrex measuring cup microwaved at 50 percent heat for 3 minutes, stirring once at 2 minute mark; or in ovenproof bowl set in 350-degree oven for 15 minutes. Whisk butter into melted chocolate, 1 tablespoon at a time; stir in salt, vanilla, and coffee or liquor until completely incorporated. Whisk in yolks, one at a time, making sure that each is fully incorporated before adding the next; set chocolate mixture aside.
Stir egg whites in clean mixing bowl set over saucepan of hot water until slightly warm, 1 to 2 minutes; remove bowl from saucepan. Beat with electric mixer set at medium speed until soft peaks form. Raise mixer speed to high and slowly add sugar; beat to soft peaks. Whisk a quarter of the beaten whites into chocolate mixture to lighten it, then gently fold in remaining whites.
Whip cream to soft peaks; gently fold into mousse. Spoon portion of mousse into six to eight individual serving dishes or goblets. Cover and refrigerate to blend flavors, at least 2 hours. (Can be covered and refrigerated up to 24 hours.) Serve with additional whipped cream.
