1. In the bowl of an electric mixer fitted with the whisk attachment, on high speed, beat whipping cream until stiff peaks form. Set aside.
2. In a small bowl, whisk together melted chocolate, coffee, and liqueur until smooth. Add powdered sugar; stir until combined.
3. Pour chocolate mixture into whipped cream. Beat lightly until mostly combined, then switch to a rubber spatula to fully incorporate.
4. Spoon or pipe mousse into small cups. If desired, top with additional whipped cream and chocolate shavings.
Mousse freezes well in an airtight container. If you plan to serve these in individual portions, it’s best to freeze it in individual jars or containers, so it defrosts and serves nicely.