Preheat the oven to 350°F (175°C). In a mixing bowl, combine the chocolate cookie crumbs and melted butter. Mix until the crumbs are evenly coated. Press the mixture firmly into the bottom of a 9-inch springform pan to form the crust. Bake for 10 minutes, then remove from the oven and let cool.

In a medium saucepan, heat the heavy cream over medium heat until it begins to simmer. Remove from heat and add the chopped semi-sweet chocolate. Let it sit for a minute, then stir until smooth and fully melted.

In a separate bowl, whisk together the eggs, granulated sugar, vanilla extract, and salt until well combined. Gradually add the chocolate mixture to the egg mixture, whisking constantly to prevent the eggs from cooking.

Pour the chocolate mousse filling over the cooled crust in the springform pan. Smooth the top with a spatula.

Refrigerate the cake for at least 4 hours, or until set. For best results, chill overnight.

Once set, carefully remove the sides of the springform pan. Top the mousse cake with whipped cream and garnish with chocolate shavings before serving.