Mix cookie crumbs, sugar, butter, and salt.

Press firmly into a parchment lined 9×13 pan.

Chill while preparing mousse.

Melt dark chocolate and butter over low heat while stirring constantly until mostly melted. Remove from heat and stir until the remaining chocolate melts. Set aside to cool slightly.

Beat mascarpone briefly, then slowly add in the cream while beating. Add in sugar, vanilla, and salt and continue to whip to stiff peaks.

3.Fold 1/3 of the whipped cream into chocolate until combined and then add in the rest of the whipped cream and fold again until combined.

Spread over crust and chill 20–30 minutes.

Melt milk chocolate (and butter if using) in a microwave safe bowl at 50% power in 30 second intervals. Stir in between each interval until 90% melted and then stir until melted completely. Set aside to cool briefly.

Beat mascarpone, then slowly add in the cream while beating. Add in sugar, vanilla, and salt and continue to whip to medium-stiff peaks.

Fold 1/3 of the whipped cream into chocolate until combined and then add in the rest of the whipped cream and fold again until combined.

Spread the milk chocolate layer evenly over the dark chocolate layer.

Chill 20–30 minutes.

Warm heavy cream over low heat until steaming and just starting to simmer. Pour hot cream over white chocolate, let sit for 1 minutes; stir until smooth. Cool and chill until slightly thickened, 3-5 minutes.

Whip remaining cream with vanilla, salt, and sugar (if using) to medium peaks.

Fold 1/3 of the whipped cream into white chocolate ganache until combined and then add in the rest of the whipped cream and fold again until combined.

Spread evenly over the milk chocolate layer. Chill 2–4 hours or overnight.

Dust the top with grated chocolate or cocoa powder.