To make the crust: Preheat the oven to 350 degrees. Crush cookies in a food processor. Stir in melted butter and espresso until combined. Press into the bottom and up the sides of an 11-inch round tart pan with a removable bottom. (Or just a regular pie plate will work.) Bake for 5 minutes, just long enough for the crust to set. Set aside and allow to cool completely.
To make the mousse: In a bowl, whisk together Kahlua, cocoa, egg yolks, espresso powder and salt. Melt chocolate and butter in a heatproof bowl set over (not in) a pan of simmering water. Whisk in liqueur mixture. Cook, whisking constantly until mixture is thickened and registers 160 degrees on an instant read thermometer. Let cool completely.
Beat cream until heavy peaks form. Gently fold in chocolate mixture. Spread into cooled tart shell. Chill overnight or at least one hour until set.