In a food processor, crush the cookies until fine. Transfer to a mixing bowl.
Melt the butter in a pan and add the chocolate. Stir until smooth.
Add to the crushed cookies and stir well.
Pour the mixture into a 9 inch spring from or deep pie pan and spread out evenly making sure to push it about 1 1/2 inches up the side of the pan.
Place the crust in the refrigerator and chill for at least 15 minutes.
For the filling, melt the chocolate and transfer to a bowl.
Add the mascarpone cheese and whisk well making sure there are no lumps.
Add the egg yolks and whisk well.
Beat the cream until soft peaks, gradually fold in the chocolate mixture making sure the whole mixture is free from streaks of cream.
Using a stand mixer, whisk the egg whites until they start to thicken.
Gradually add the sugar until the mixture thickens to meringue style soft peaks.
Fold the meringue into the mousse.
Transfer the mousse to the chilled cookie crust, spreading it out and making it even.
Chill the tart in the refrigerator for at least 3 hours or overnight.
Melt the dark chocolate and white chocolate. Pour the dark chocolate onto a marble slab or large cookie tray. Spread it out thinly.
Do the same with the white chocolate and drizzle it over the dark chocolate.
using a spatula spread the white chocolate through the dark chocolate mixing the two together.
Make sure to spread it thinly, this way it'll dry faster.
Once dry to the touch, take a large chopping knife and scrape the chocolate creating shavings and curls. Work quickly before it starts to melt. Transfer the shaved pieces to a plate and keep in the fridge until ready to place on top of the mousse tart.
Bring the tart to room temperature 1 hour before serving.