Steps:

• Heat oven to 350 degrees. Add Oreos to the bowl of a food processor and process until fine crumbs form. Add melted butter and salt, and process until crumbs are evenly moistened. Tip mixture out into a 9-inch pie plate, and press crumbs evenly into bottom and sides of pan. Bake until crust is fragrant and set, about 12 minutes. Transfer to a rack to cool.
• Set a fine mesh sieve over a medium bowl. In a medium saucepan, whisk together granulated sugar, cornstarch, cocoa powder and salt. Pouring just a little at first, whisk the milk into the sugar mixture until you have a smooth paste. Gradually add remaining milk, whisking constantly. Whisk in egg yolks. Cook mixture over medium heat, whisking constantly, until it has thickened and just come up to a low boil, about 8 to 9 minutes. Continue to cook, stirring constantly, for 1 minute.
• Pour custard through sieve, pushing it through with a rubber spatula. Discard any solids. Stir in chocolate, butter and vanilla, and let it stand for 1 minute. Stir until smooth. Pour mixture into prepared crust. Let cool slightly, then cover loosely with plastic wrap and chill until firm, at least 6 hours.
• To serve, whip cream and confectioners’ sugar until soft peaks form. Top with whipped cream and sprinkle with crushed Oreos, if using.

Steps:

• Preheat oven to 350°F. Butter 9-inch-diameter springform pan with 2 3/4-inch-high sides. Finely grind cookies in processor. Add butter and process until mixture is evenly moistened. Press crumb mixture onto bottom and up sides of prepared pan to form thin crust. Bake crust 5 minutes. Transfer crust to rack and cool completely.
• Combine chocolate, vanilla and salt in processor. Bring 1 cup cream to boil in heavy small saucepan. With processor running, gradually pour hot cream through feed tube and process until chocolate is melted and smooth. Transfer mixture to large bowl. Cool to room temperature, stirring occasionally.
• Beat 2 cups cream and sugar in large bowl to stiff peaks. Fold into chocolate mixture. Pour mousse into prepared crust. Chill until set, about 6 hours. (Can be prepared 1 day ahead.)
• Beat remaining 3/4 cup cream in medium bowl to firm peaks. Transfer to pastry bag fitted with medium star tip. Pipe rosettes of cream around edge of cake. Garnish with chocolate shavings.