Melt the chocolate chips in a 2.5-qt. sauce pan over very low heat, stirring constantly.
Add the coffee; stir in the liqueur and orange zest. Let the mixture cool to room temperature.
Add the egg yolks, one at a time, beating thoroughly after each addition.
In a cold mixing bowl, whip 1 c. cream until thickened. Gradually add the sugar, beating until the cream is stiff.
In another mixing bowl, beat the egg whites with the salt until they form stiff peaks. Gently fold the egg whites into the whipped cream.
Stir about 1/3 of the cream mixture thoroughly into the chocolate mixture. Scrape the remaining cream mixture over the chocolate base and gently fold them together. Pour the mousse into individual dessert cups or a serving bowl and refrigerate until set (about 2 hours).
At serving time, whip the remaining 1 c. cream until thickened. Add the vanilla and whip to soft peaks.
Top each portion of mousse with a dollop of whipped cream.