1. Crust: Preheat oven to 375 degrees. In a food processor, pulse wafers until finely ground (you should have 1 1/2 cups). Add butter, granulated sugar, and salt; pulse to combine. Firmly press mixture into bottom and up sides of a 9-inch pie dish. Freeze 15 minutes. Bake until dry, about 12 minutes. Let cool completely.

2. Filling: In a medium saucepan, whisk together granulated sugar, cornstarch, salt, vanilla, and cardamom. Whisk in egg yolks, then milk. Bring to a boil over medium-high heat, whisking constantly, then cook 2 minutes more. Remove from heat and whisk in chocolate until melted, then butter. Pour filling into crust; let stand 30 minutes. Refrigerate 1 hour. Press plastic wrap directly onto surface; refrigerate until set, at least 2 hours more and up to 2 days.

3. Topping: Whisk cream with confectioners' sugar and vanilla to medium peaks. Dollop on top of pie. Dust with cocoa; serve immediately. (For cleanest cuts, wipe knife between slices.)