Chocolate Mousse Pie

Place a rack in lower third of oven; preheat to 350°. Butter a 9" springform pan with 2¾" sides. Finely grind 21 chocolate sandwich cookies (such as Oreos) in a food processor. Add ¼ cup (½ stick) unsalted butter, cut into pieces, room temperature, and process until mixture is evenly moistened. Press mixture into bottom and up sides of prepared pan to form a thin crust. Bake 5 minutes. Transfer pan to rack and let pie crust cool completely.

Combine 12 oz. semisweet chocolate, finely chopped, 1 tsp. vanilla extract, and a pinch of kosher salt in food processor (no need to wipe it out). Bring 1 cup heavy cream just to a boil in a small heavy saucepan. With processor running, gradually pour hot cream through feed tube and process until chocolate is melted and smooth. Transfer chocolate mixture to a large bowl. Cool to room temperature, stirring occasionally.

Beat 2 cups heavy cream, chilled, and ¼ cup sugar in a large bowl until stiff peaks form. Fold into chocolate mixture. Pour mousse into cool prepared crust. Chill until set, about 6 hours. (Can be prepared 1 day ahead.)

Beat remaining ¾ cup heavy cream, chilled, in a medium bowl to medium-firm peaks. Transfer to pastry bag fitted with medium star tip. Pipe rosettes of cream all around top of pie. Garnish with chocolate shavings (if using).