For the chocolate crust:

1. Preheat oven to 350ºF/180°C.
2. Mix crumbs with butter until it looks like wet sand.
3. Place mixture in a 9-inch pie pan with removable bottom. Carefully press evenly against bottom and sides of the pan.
4. Bake for 10 minutes. Let cool on a wire rack completely.

For the mousse filling:

1. Combine sugar and cornstarch in a small saucepan.
2. Add milk, mix well until cornstarch is dissolved and cook on medium heat until the mixture reaches the boiling point. Stir frequently to avoid lumps.
3. Boil for 1 minute, whisking constantly. It will thicken. 
4. Remove from heat and immediately add chocolate and mint extract. Combine well. The mixture will look like jello gone bad, don't worry.
5. Cover with plastic wrap touching the surface and let cool to room temperature. You can speed this process by refrigerating it.
6. Beat chocolate mixture a bit with an electric mixer until it starts to soften.
7. Add cream and slowly beat until smooth, about 3 minutes.
8. Pour the filling into the prepared crust and spread evenly.
9. Refrigerate until ready to serve, at least 2 hours.
10. Sprinkle with crushed mint candies right before serving.