Mix all ingredients together in a food processor until it becomes a semi smooth paste and set aside.

Position rack in center of oven and preheat to 350°F.

Lightly butter bottom and sides of 13x9x1-inch baking pan. Line bottom of pan with parchment paper.

Lightly dust sides with flour.

Sift 1/2 cup flour, cocoa powder, baking powder, and salt into medium bowl.

Using electric mixer, beat eggs and egg yolks in another medium bowl until pale and foamy, about 2 minutes. Add sugar and vanilla; beat until mixture is thick and ribbon forms when beaters are lifted, about 3 minutes.

Mix in hazelnut paste gently.

Sift flour mixture over egg mixture and gently fold in with rubber spatula just until incorporated.

Transfer batter to baking pan; spread evenly to edges.

Bake until top springs back when gently touched and tester inserted into center comes out clean, about 15 minutes.

Transfer baking pan to rack and cool cake 20 minutes. Run small knife around sides of cake to loosen.

Invert cake onto parchment-lined work surface; gently peel off parchment. Cool completely.

Using serrated knife, cut cake in half lengthwise, each half should be 12×4-inch rectangle; trim if necessary.

Position rack in center of oven and preheat to 175°F. Using dark pen or pencil, trace two 12×4-inch rectangles on sheet of parchment paper; turn parchment over (rectangles will show through) and place on large rimmed baking sheet. Trace one 12×4-inch rectangle on second sheet of parchment paper; turn parchment over and place on second rimmed baking sheet.

Sift powdered sugar, superfine sugar, and salt into medium bowl.

Combine egg whites and cream of tartar in large bowl of heavy-duty stand mixer fitted with wire whisk; beat on medium-low speed until frothy.

Increase speed to medium-high and beat until soft peaks form.

Increase speed to high; gradually add sugar mixture and beat until stiff and glossy peaks form, about 3 minutes.

Beat in vanilla and Frangelico.

Divide meringue equally among prepared rectangles on parchment (about 1 1/3 cups for each); spread meringue evenly within traced lines.

Bake meringues until dry and crisp but not browned, about 3 hours. Turn off oven and let meringues cool completely in oven.Carefully remove meringue rectangles from parchment.

Simmer cream, butter, and corn syrup in medium saucepan. Remove from heat. Add chocolate; whisk until smooth.

Whisk in vanilla and salt. Let stand at room temperature until thick enough to spread, 1 hour to 1 1/2 hours.

Heat the water and sugar until it starts to boil and the sugar is dissolved. Cool

Add 1 Tbsp of Frangelico to sugar water to make Frangelico Glaze.

Transfer 1/2 cup buttercream to pastry bag fitted with medium star tip; reserve for piping rosettes.

Brush off any crumbs from meringue layers and cake layers. Place several small dabs of buttercream down center of 15-inch-long rectangular serving platter.

Place 1 meringue layer, top side up, atop buttercream on platter, anchoring to platter.

Using small offset spatula, spread 2/3 cup buttercream over meringue.

Top with 1 cake layer, top side down, pressing gently to adhere. Brush cake layer with 1/3 cup of the Frangelico syrup (cake layer will be very moist). Spread 1/2 cup buttercream over the cake layer.

Top with second meringue layer, top side up. Spread 1/2 cup buttercream over.

Sprinkle 1/4 cup of toasted hazelnuts on top of buttercream.

Cover with second cake layer, top side down. Brush with 1/3 cup Frangelico syrup and then spread layer with 1/2 cup buttercream.

Top with third meringue layer, bottom flat side up. Spread thin layer of buttercream over top and sides of cake.

Finish cake with remaining 1/4 cup of toasted hazelnuts and food glitter if desired.

Chill cake at least 6 hours.