Preheat the oven to 350°F. Grease a 9-inch springform pan.

In a medium bowl, combine the chocolate cookie crumbs, melted butter, 1/4 cup granulated sugar, and salt. Mix until the crumbs are evenly coated. Press the mixture firmly into the bottom of the prepared springform pan to create a crust.

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

In a large mixing bowl, whip 1 cup of heavy cream with the powdered sugar and vanilla extract until soft peaks form. Set aside.

In another bowl, beat the egg yolks with 1/4 cup of granulated sugar until pale and thick. Gradually stir the melted chocolate into the egg yolk mixture until well combined.

Gently fold the whipped cream into the chocolate mixture until no white streaks remain.

In a clean bowl, beat the egg whites until soft peaks form. Gradually add the remaining 1/2 cup of granulated sugar and continue beating until stiff peaks form. Gently fold the egg whites into the chocolate mixture in three additions.

Pour the mousse mixture over the crust in the springform pan, smoothing the top. Refrigerate for at least 4 hours or until set.

Before serving, whip the remaining 1 cup of heavy cream until soft peaks form. Spread or pipe the whipped cream over the top of the chilled mousse cake.