Make the nut meringue: Position a rack in the center of the oven and preheat to 350°F. Spread the almonds on one half of a baking sheet and the hazelnuts on the other half. Bake the nuts until lightly toasted, about 10 minutes. Set aside to cool.

Coat the bottom and sides of a 12- by 18-inch baking sheet with nonstick spray and line the bottom with parchment. Spray the parchment generously, then dust with flour and tap out any excess. Set aside.

Rub the hazelnuts between your hands or in a kitchen towel to loosen and remove the skins. In a food processor fitted with a metal blade or in a blender, pulverize the hazelnuts, almonds, sugar, and cornstarch, then transfer to a bowl.

In a stand mixer fitted with the whisk attachment, beat the egg whites on low until frothy, about 2 minutes. Add a pinch of salt, turn the speed to high, and beat until the whites hold stiff peaks, about 4 minutes. Gradually fold in the ground nut mixture over the egg whites. Scrape the batter onto the prepared baking sheet and smooth with a spatula. Bake until light golden brown, 20–25 minutes. Set aside to cool completely. (The nut meringue can be made up to 3 days in advance and covered with plastic wrap.)

Make the praline: Place the almonds on a baking sheet and bake until lightly toasted, about 10 minutes. Set aside to cool, then coarsely chop.

Lightly coat a second baking sheet with nonstick spray or line it with a silicone baking mat and set aside. In a medium heavy skillet over medium heat, spread the sugar in an even layer. Cook, without stirring, until the sugar begins to melt on the bottom or around the edges, about 2 minutes. Using a heatproof spatula, slowly drag the liquefied sugar to the center and stir gently until all of it has melted. Continue cooking, stirring occasionally, until the color is deep amber and it begins to foam a bit, about 1 minute more. Remove from the heat and immediately stir in the almonds, then pour onto the prepared baking sheet and spread in an even layer. Set aside to cool completely, about 10 minutes.

Break up the praline with your hands. In a food processor fitted with a metal blade or with a chef’s knife, chop the praline into very small pieces. Transfer to a medium bowl. (The praline can be made 1 week in advance and stored in an airtight container.)

Make the chocolate ganache: To a medium heatproof bowl, add the chocolate. To a small pot over medium heat, add the crème fraîche and cook until it just comes to a boil, then pour over the chocolate. Set aside for 1 minute, then stir until the mixture is completely smooth. Set aside to cool.

Make the praline and vanilla creams: To a stand mixer fitted with the whisk attachment, add the crème fraîche, cream, sugar, and vanilla and beat on medium until the mixture is stiff but still glossy, about 1½ minutes. Don’t overbeat.

For the praline cream, stir ⅔ cup of the whipped crème fraîche mixture into the chopped praline. (The mixture will seem somewhat stiff but the ingredients will meld, and by the time you’re ready to use it, the praline cream will be spreadable.) Set aside.

For the vanilla cream, add the cognac to the remaining crème fraîche mixture and whisk lightly until firm. Set aside.

To assemble the marjolaine, run a knife around the edges of the meringue to loosen it from the baking sheet, then place another baking sheet over the top and invert the meringue. Peel away the parchment, holding down the meringue as you pull to keep it from breaking. (If you do break the meringue, it can be patched together when you are assembling the layers.)

Using a serrated knife, cut the meringue crosswise (not lengthwise) into four even rectangles, each about 4½ by 12 inches. Cover a baking sheet or large rectangular platter with plastic wrap and set one meringue rectangle on top. Spread ¾ cup of the chocolate ganache over the top in an even layer. Top the ganache layer with a second meringue rectangle and spread evenly with the vanilla cream. Top with a third meringue rectangle and spread evenly with the praline cream. Top with the last meringue rectangle. Cover the marjolaine in plastic wrap and refrigerate for at least 8 hours. (The assembled unfrosted cake can be stored in the refrigerator for up to 3 days.) Cover the remaining ganache and refrigerate.

To finish the cake, trim the rough edges with a serrated knife. Bring a small pot of water just to a simmer, place the bowl of ganache over it, and heat until spreadable. Spread evenly over the top and sides of the cake. Using the serrated knife, slice the marjolaine crosswise into slabs, dipping the blade in hot water and wiping it clean after each cut. Serve at room temperature with lightly sweetened whipped cream if desired.