How to Make It
In a medium saucepan, bring 2 cups of the heavy cream just to a simmer over medium-high heat. Remove from the heat and add the chocolate chips. Let stand for 2 minutes, then whisk until smooth. Whisk in the cocoa powder, instant espresso, and salt. Transfer the chocolate cream to a large bowl and let cool completely. Cover with plastic wrap and refrigerate until chilled, about 1 hour.
Using a handheld mixer, beat the chocolate cream until smooth and holds stiff peaks when you lift the beater out of the bowl. Do not overbeat the cream because the fat will begin to separate and it’ll look curdled (see Note)—you’re making mousse, not butter! Spoon the chocolate mousse into bowls or glasses and refrigerate until ready to serve.
In another large bowl, using the handheld mixer, beat the remaining ½ cup heavy cream with the vanilla until smooth and holds a soft peak when you lift the beater out of the bowl. Spoon the whipped cream on top of the chocolate mousse and sprinkle with shaved or grated chocolate. Serve cold.
If you happen to whip the cream a little too much, very gently fold in cold heavy cream, 1 tablespoon at a time, until the mixture is creamy again.