Preheat the oven to 350°F (175°C). Grease a 9-inch springform pan and set aside.

In a medium bowl, combine the chocolate cookie crumbs, melted butter, granulated sugar, and salt. Mix until well combined. Press the mixture firmly into the bottom of the prepared springform pan to form a crust.

Bake the crust in the preheated oven for 10 minutes. Remove from the oven and let it cool completely.

In a heatproof bowl, melt the chopped semisweet chocolate over a pot of simmering water, stirring until smooth. Remove from heat and let it cool slightly.

In a separate bowl, beat the egg yolks with the powdered sugar and vanilla extract until pale and creamy. Gradually add the melted chocolate to the egg yolk mixture, stirring until well combined.

In another clean bowl, beat the egg whites until stiff peaks form. Gently fold the egg whites into the chocolate mixture until no white streaks remain.

In a separate bowl, whip the heavy cream to stiff peaks. Gently fold the whipped cream into the chocolate mixture until fully incorporated.

Pour the chocolate mousse over the cooled crust in the springform pan. Smooth the top with a spatula. Cover and refrigerate for at least 4 hours or until set.

Before serving, garnish with chocolate shavings or a dusting of cocoa powder if desired. Carefully remove the sides of the springform pan and slice the cake.