1. Add the chopped chocolate and 1 cup of the heavy whipping cream to a small saucepan. Heat on low, stirring often, until the chocolate is completely melted and mixture is steaming. Take care not to boil. (This step can also be done in the microwave. Heat in 30-second increments at 50% power, stirring in between.)
2. Whisk in the instant espresso powder and set the chocolate aside to cool to room temperature, stirring once or twice as it cools.
3. In a mixing bowl, use a hand-held mixer or an immersion blender with a whisk attachment to whip together the remaining cup of heavy whipping cream until soft peaks form.
4. Add the powdered sugar, cornstarch, and vanilla extract to the whipped cream and continue to whip until firm, but not stiff, peaks form.
5. Take 1 cup of the whipped cream mixture and gently fold it into the cooled chocolate just until combined. Be careful not to over-mix here.
6. Spoon the chocolate mousse into 4 to 6 (3 to 4 ounce) serving glasses or ramekins and place in the refrigerator to chill for at least 2 hours, or overnight.
7. Continue to beat the remaining whipped cream mixture until stiff peaks form. Cover and refrigerate until ready to serve.
8. To serve, top each mousse cup with a generous dollop of the remaining whipped cream and some chocolate shavings.