Mix cookie crumbs and melted butter. Press thickly up sides and then onto bottom of a 9 to 10-inch tart pan with removable bottom. Bake 10 minutes. Cool to room temperature and then place in freezer.
Prepare filling. In medium saucepan, whisk sugar, cornstarch, espresso powder and salt.
In a bowl, whisk milk and egg yolks until well blended. Pour milk mixture into saucepan whisking well.
Bring mixture to a boil over medium heat, whisking constantly. Boil for 1 minute. Remove from heat.
Drop chocolate squares on to surface of hot filling; let stand 30 seconds.
Add butter and flavoring and whisk until chocolate and butter are melted and mixture is smooth.
Pour into well-chilled pie shell. Cover surface of pie with plastic wrap to avoid skin formation.