CHOCOLATE CRUST

1. Adjust an oven rack to the middle of the oven. Preheat the oven to 350°f / 180°c. 
2. Grease a 9" x 13" (23 x 33cm) pan and line it with a parchment sling.
3. Place cookies in the bowl of a food processor and process until broken down into fine crumbs. Move the crumbs to a medium bowl, and pour the melted butter over the top. Use a spatula to stir together until combined.
4. Press the mixture onto the bottom of the prepared pan and bake for 10 minutes. Remove the pan from the oven and let cool.

CHOCOLATE CAKE

1. Keep the oven temperature at 350°f / 180°c.
2. In a small saucepan, melt the chocolate and butter together over low heat, stirring frequently, until smooth. Remove from the heat and stir in the cocoa powder. 
3. In a large bowl, whisk together the sugar, eggs, vanilla, and salt until smooth. Add the flour and mix again until combined. Add the warm chocolate and whisk into the batter until combined. Let the mixture sit for 15 minutes. 
4. Pour the cake batter over the cooled, baked crust and use an offset spatula to smooth the top. 
5. Bake the cake until the edges are set and the center jiggles slightly, 15 to 18 minutes. Remove from the oven and let cool completely on a wire rack. Cover the pan with plastic wrap and refrigerate for 4 hours, or overnight.

CHOCOLATE MOUSSE

1. Heat 240g of the heavy cream in a small, heavy saucepan until hot. 
2. In a medium saucepan off the heat, whisk the egg yolks. Whisking constantly, slowly add the sugar to the egg yolks, then the salt, and then slowly pour in the warmed heavy cream. Cook over medium heat, stirring constantly, until the mixture registers 160°f / 70°c. Pour the mixture through a fine-mesh sieve into a large bowl and stir in the vanilla. 
3. In a small saucepan over low heat, melt the chocolate, stirring frequently until smooth. Whisk the chocolate into the custard until smooth, then let cool.
4. In the bowl of a stand mixer fitted with the whisk attachment (or just using a mixing bowl and whisk), beat the remaining 360g heavy cream until stiff peaks form. Whisk one-third of the whipped cream into the chocolate custard to lighten it, then gently fold in the remaining whipped cream.
5. Transfer the mousse to a large bowl and cover the bowl with plastic wrap. Refrigerate for 4 hours, or overnight.

WHIPPED CREAM

1. Ten minutes before whipping the cream, place the bowl and whisk from a stand mixer  (or a hand whisk) in the freezer and let chill. In the chilled bowl, whisk together the heavy cream, sugar, vanilla, and salt for 30 to 45 seconds. Increase the speed to medium and beat for 30 to 45 seconds. Increase the speed to high and beat until the cream is smooth, thick, and nearly doubled in volume, 30 to 60 seconds. The whipped cream can be made 2 hours ahead and stored in the fridge.

ASSEMBLY

1. Transfer the chilled chocolate mousse onto the top of the baked, cooled cake. Use an offset spatula to even the top. return the bars to the fridge and let chill for 1 hour.
2. When ready to serve, cover the bars with the whipped cream. Coat the top with grated chocolate, if desired. Use the parchment sling to remove the cake from the pan, and cut into slices. Bars can be stored in the fridge in an airtight container for up to 5 days.