1. Crust: Preheat oven to 375 degrees. Toast hazelnuts in oven on a rimmed baking sheet until skins split and flesh turns deep golden brown, 10 to 12 minutes. When cool enough to handle, rub nuts in a clean kitchen towel to remove skins (some will remain). Let cool completely. Finely grind in a food processor. Reduce oven temperature to 325 degrees.

2. Stir together nuts, cookie crumbs, espresso powder, salt, and butter. Press onto bottom and up sides of an 11-inch round tart pan with a removable bottom. Refrigerate until firm, about 30 minutes.

3. Bake tart shell on a baking sheet until firm to the touch, about 20 minutes. Let cool completely on a wire rack.

4. Mousse: Whisk liqueur, cocoa, egg yolks, espresso powder, and salt in a medium bowl. Melt chocolate and butter in a heatproof bowl set over a pan of simmering water. Whisk in liqueur mixture. Cook, whisking, until mixture is thickened and registers 160 degrees on an instant-read thermometer. Let cool completely.

5. Beat cream just to stiff peaks; set aside. Whisk egg whites and sugar in the heatproof bowl of an electric mixer set over a pan of simmering water. Cook, whisking, until sugar is dissolved and mixture registers 140 degrees on thermometer.

6. Fit mixer with the whisk attachment; transfer bowl with meringue to mixer. Starting on medium speed and gradually increasing to high, beat meringue until shiny and cool. Using a rubber spatula, fold chocolate mixture and then whipped cream into meringue. Spread in tart shell. Refrigerate until set, about 1 hour.