Preheat oven to 325 degrees F.

Make Crust

In a food processor, whirl cookies and hazelnuts until finally ground. Add butter and whirl until crumbs start to clump together. Press crumbs over bottom and up sides of a 9-1/2 inch tart pan with a removable rim.

Bake until crust is set, about 10 minutes. Let cool completely.

Chocolate Hazelnut Spread

Whirl 1 cup skinned roasted hazelnuts in a food processor, scraping bowl as needed, until it looks like peanut butter. Melt 4 ounces semisweet chocolate (good quality) in a small saucepan over low heat, stirring often; let cool slightly. Add melted chocolate, 2 tablespoons unsweetened cocoa powder, 1/2 cup powdered sugar, 1 teaspoon Rain’s Choice vanilla extract and a few pinches of sea salt to food processor, and whirl until smooth. Some fine nutty bits will remain. Spread chocolate hazelnut spread over crust. Chill until spread is firm, about 30 minutes.

Melt chocolate and sugar with 2 tablespoons water in a small saucepan over low heat until smooth, stirring frequently, about 10 minutes. Set aside.

Beat egg whites in a medium bowl using a mixer until they hold stiff peaks. Beat cream in another bowl until stiff. Stir yolks and vanilla into chocolate mixture.

Gently fold chocolate mixture into egg whites, then fold in cream until no streaks remain. Pour mousse into crust and spread evenly. Sprinkle with chopped nuts and chill until mousse is firm, at least 3 hours and up to a day ahead.

Remove from pan and cut into wedges.