Place the chocolate, confectioners' sugar, vanilla, and salt (if using) in a microwave-safe bowl. Heat at half-power in the microwave in 30-second intervals, stirring after each interval. Repeat until the chocolate melts when stirred. Stir until smooth. (You can melt in a double boiler over medium-low heat if you prefer.) Allow the chocolate mixture to cool for about 10 minutes.
Using an electric mixer with a whisk attachment, whip the cream to firm peaks.
Add about 1/4 of the whipped cream to the cooled chocolate mixture, and fold just until combined.
Gently fold the remaining whipped cream into the chocolate mixture until combined. Be sure not to over-mix.
Divide the mousse among 4 to 6 individual glasses. Serve immediately, or cover and refrigerate until ready to serve. Add any toppings you like.
Notes
*I usually use about 70% cacao. Chocolate baking bars will melt more easily than chocolate chips.
Store covered in the refrigerator up to 5 days. (It will be at its best within the first day or two.)