In a food processor, combine chocolate, egg, and vanilla. In a small pot, heat the cream to a slow boil, then slowly add the heated cream to the chocolate mixture: careful not to break the mix. Transfer this to a container and refrigerate for 1 to 3 hours.
In a small pan heat 3 tablespoons of butter. Pulse the cookies in a food processor for about 20 seconds and add 1 tablespoon of the solid butter at a time. Slowly add the melted butter until the crust becomes shiny and easy to work with. Line 1 (6-ounce) cup with plastic wrap and begin forming your crust in the cup about 1/2-inch thick.
Refrigerate for 10 minutes. Begin filling with the mouse until it is up to the edge of the crust. Refrigerate for 1 hour and invert onto a dish. Remove plastic and serve with chocolate shavings.