1. Preheat oven to 350°. Make crust: Roast hazelnuts on a rimmed baking sheet until light golden, about 18 minutes. Rub the warm nuts in a kitchen towel to remove skins. Reduce oven temperature to 325°.
2. Whirl cookies and hazelnuts in a food processor until finely ground. Add butter and whirl until crumbs start to clump together. Press crumbs over bottom and up sides of a 9 1/2-in. tart pan with a removable rim.
3. Bake until crust is set, about 10 minutes. Let cool completely. Spread chocolate-hazelnut spread over crust. Chill until spread is firm, about 30 minutes.
4. Make mousse: Melt chocolate, sugar, and 2 tbsp. water in a small saucepan over low heat until smooth, stirring frequently, about 10 minutes. Set aside.
5. Beat eggs whites in a medium bowl using a mixer until they hold stiff peaks. Beat cream in another bowl until stiff. Stir yolks and liqueur into chocolate mixture.
6. Fold chocolate mixture gently into egg whites, then fold in cream until no streaks remain. Pour mousse into crust and spread evenly (if using pasteurized egg whites, chill 20 minutes at this point). Sprinkle with chopped hazelnuts and chill until mousse is firm at least 3 hours and up to 2 days. Remove rim from pan and cut into wedges.
*Hazelnuts from Oregon--where virtually all the U.S. crop grows--taste much sweeter and fresher than imports. Find them in bulk bins in grocery stores (check the country of origin) and at freddyguys.com, hazelnuthill.com, or yournw.com. This dessert contains raw eggs. You can substitute pasteurized egg whites for both the fresh yolks and whites if you are concerned about eating raw eggs; follow the manufacturer's conversion amounts.
Note: Nutritional analysis is per slice.