Preheat your oven to 350°F (175°C). In a medium bowl, mix together the chocolate cookie crumbs and melted butter until fully combined. Press this mixture firmly into the bottom of a 9-inch springform pan. Bake for 10 minutes, then let it cool completely.

Melt the dark chocolate in a heatproof bowl over simmering water. Let it cool slightly. In a separate bowl, whip the cream, powdered sugar, and vanilla extract until soft peaks form.

In another bowl, beat the egg whites until stiff peaks form. Gently fold the whipped cream into the cooled chocolate, then fold in the egg whites until combined.

Transfer the mousse onto the cooled crust, smooth the top, and refrigerate for about 30 minutes.

Repeat the same melting and whipping process with the milk chocolate. After the dark chocolate mousse has set for 30 minutes, pour this layer over the dark chocolate mousse and return to the fridge for another 30 minutes.

Finally, repeat the melting and whipping process with the white chocolate. Once the milk chocolate layer has set, pour the white chocolate mousse on top. Smooth it over, cover the cake with plastic wrap, and refrigerate for at least 4 hours or overnight for best results.

Once fully chilled, run a knife around the edges of the springform pan, then carefully release the springform. Slice and serve chilled, garnishing with chocolate shavings if desired!