Preheat oven to 350°F (180°C). Spray a 9-inch pie plate with baking spray with flour.
For crust: Separate cookies; scrape off and discard filling. In the work bowl of a food processor, pulse cookies until finely ground.
In a medium bowl, whisk together cookie crumbs, ground hazelnuts, granulated sugar, and salt. Stir in melted butter until combined. Press mixture into bottom and up sides of prepared pan.
Bake until fragrant, 15 to 20 minutes. Let cool completely.
For filling: In the heatproof bowl of a stand mixer, whisk together granulated sugar and eggs until frothy. Place bowl over a small saucepan of simmering water. Cook, whisking constantly, until sugar dissolves, mixture is warm to the touch, and a candy thermometer registers 160°F (71°C), 2 to 4 minutes. Whisk in cocoa and salt.
Carefully return bowl to stand mixer. Using the whisk attachment, beat at high speed until mixture is doubled in size and bottom of bowl is cool to the touch, 4 to 5 minutes.
Meanwhile, in a small microwave-safe bowl, place 1 tablespoon (15 grams) water; sprinkle gelatin onto water, and let stand for 1 minute. Heat gelatin mixture on high in 10-second intervals, stirring between each, until it comes to a boil and looks clear (about 30 seconds total). Let cool.
Reduce mixer speed to low; gradually add hazelnut chocolate spread, beating until well combined. Add liqueur and vanilla, beating until well combined. Add butter, 1 tablespoon (14 grams) at a time, beating until well combined. Add mascarpone, 1 tablespoon (14 grams) at a time, beating until well combined after each addition. Add gelatin mixture, and beat until well combined. Strain mixture through a fine-mesh sieve into prepared crust, stirring with a spatula to pass all mixture through sieve. Refrigerate, uncovered, until thick, cold, and set, at least 6 hours, or up to overnight.
For topping: In the bowl of a stand mixer fitted with the whisk attachment, beat cold cream and confectioners’ sugar at medium-high speed until stiff peaks form; beat in vanilla. Spread onto pie, and garnish with chopped hazelnuts, if desired. Serve immediately. Cover and refrigerate for up to 2 days.