Preheat the oven to 350°F. Grease a 9-inch springform pan and line the bottom with parchment paper.

In a medium saucepan, melt the semi-sweet chocolate over low heat, stirring until smooth. Remove from heat and let cool slightly.

In a separate bowl, whip 1/2 cup of heavy cream until soft peaks form. Set aside.

In another bowl, beat the egg yolks with sugar until pale and creamy. Stir in the melted semi-sweet chocolate and vanilla extract.

Gently fold the whipped cream into the chocolate mixture until fully combined. Set aside.

In a clean bowl, beat the egg whites with salt until stiff peaks form. Carefully fold the egg whites into the chocolate mixture in three additions.

Pour the mousse into the prepared springform pan and smooth the top. Refrigerate for at least 2 hours to set.

For the white chocolate mousse, melt the white chocolate in a saucepan over low heat. Let cool slightly.

In a bowl, whip the remaining 1/2 cup of heavy cream until soft peaks form. Fold the whipped cream into the melted white chocolate until smooth.

Once the dark chocolate mousse has set, pour the white chocolate mousse over it and smooth the top. Refrigerate for another 2 hours.

For the crust, combine the chocolate cake crumbs, cocoa powder, and melted butter in a bowl. Press the mixture into the bottom of the springform pan over the white chocolate mousse layer.

Chill the cake for an additional hour before serving. Garnish with chocolate shavings or curls if desired.