FOR THE CRUST
16 chocolate chip cookies (3 inches; baked until crisp), finely ground (2 cups)
2 tablespoons unsalted butter melted
FOR THE FILLING
1 cup heavy cream
8 ounces semisweet chocolate chopped (1 1/3 cups)
1 large egg
1/4 cup whole milk
1 tablespoon unsalted butter
Creme fraiche, for serving
Preheat oven to 350 degrees. Make the crust: Stir together cookie crumbs and melted butter. Press into bottom and up sides of a 9 1/2-inch fluted tart pan with a removable bottom. Refrigerate until chilled, about 30 minutes.
Bake until firm and darkened, 10 to 12 minutes. Let cool completely on a baking sheet. Meanwhile, make the filling: Heat cream in a saucepan until just starting to boil. Pour over chocolate in a bowl. Let stand for 1 minute, then stir. Whisk in egg, and butter. Pour into cooled crust.
Reduce oven temperature to 300 degrees. Bake tart until edges of filling are set but center still jiggles slightly, 20 to 24 minutes. Transfer to a rack. Serve warm or at room temperature. Top with creme fraiche.