Separate the egg whites from the egg yolks. Cover the bowl with egg yolks with plastic film and refrigerate. Keep the whites at room temperature (uncovered) for 2 hours. Then, warm the mixer bowl by rinsing it with hot water and dry it. Add the egg whites to the bowl and warm them to 75-76°F (23-24°C), checking with a cooking or infrared thermometer.

Preheat the oven to 300°F (150°C). Grind the roasted hazelnuts into coarse crumbs in short pulses using a food processor. Alternatively, you can utilize a knife to chop hazelnuts. Mix the prepared nuts, 1 1/8 cup (225 g) of sugar (#1), and sifted all-purpose flour.

Place the egg whites in the metal bowl of a stand mixer fitted with a whisk attachment. Beat at a low speed, gradually increasing it to medium. Once you see the first small bubbles, start adding sugar.

Add the remaining sugar #2 (1/4 cup or 50 g) in two additions, whisking well for 30 seconds after each addition. Add the vanilla sugar and continue whisking at medium or high speed for 4-5 minutes. The meringue will become glossy with stiff peaks and hold its shape on the beaters with a stable bird's beak.

Pour the dry ingredients into the whisked egg whites in 3 additions, and gently mix with a rubber spatula from the bottom to the top. Try to work in one direction: from the center of the bowl to the sides, turning the bowl clockwise with the other hand. Do not overmix: mix to combine!

Place two cake rings on a baking sheet lined with parchment paper: one 7-inch (18 cm), the other 8-inch (20 cm) in diameter. Do not grease the sides of the rings. Divide the meringue mixture between two cake molds, ensuring the meringue heights are equal. Smooth the meringue's surface with a spatula. Use two clean toothpicks to check the height of the meringue in both rings.

Bake in the preheated oven for about 2 hours until the meringue layers are slightly browned. Do not open the oven door for the first 90 minutes.

Remove the cakes from the oven and allow them to cool still in the cake rings. Leave them uncovered at room temperature for 12-24 hours (still in the cake rings!)

The next day, use a knife to remove the cake layers, and carefully peel off the parchment paper. Place the smaller cake disk on top of the larger cake disk and cut to a uniform size. You will have two cake discs with a diameter of 7 inches (18 cm) and a thickness of 1.2 inches (3 cm). Collect the scraps and crush them into crumbs by hand.

To make the Charlotte cream, pour milk into a heavy-bottomed medium saucepan and add sugar. Bring it almost to a boil.

Remove the remaining yolks from the refrigerator and whisk them together. If the yolk surface has dried out, pass them through a fine-mesh sieve. Pour the hot milk syrup over the egg yolks in a thin stream, stirring constantly to prevent the yolks from curdling.

Bring the mixture back into the saucepan and place it on medium heat. Cook until thickened, constantly stirring vigorously. You should get the cooked mixture with the consistency of thin condensed milk.

Remove it from the heat and transfer it to a shallow enough plate to prevent the cream from continuing to cook. Cover the cream with plastic wrap and leave it to cool at room temperature.

Beat softened butter with vanilla sugar until whitish. Add the cooled custard to the butter (1 tablespoon at a time). Make sure that the butter and custard base are at the same temperature. Otherwise, the cream risks stratifying.

To assemble the cake, divide the cream into two equal parts. Add cognac to one part of the cream and whisk again with a mixer. Take about 3 tablespoons of the cream for the decoration. Sift cocoa powder over the remaining cream and mix with an electric mixer.

Apply about a teaspoon of the white cream to a cake board or serving plate to prevent the cake from slipping. Place the first cake layer, smooth side down (the bottom of the cake on the cake board). Spread the white cream on the surface of the cake. Do not use the cream left for garnish!

Place the top layer, smooth side up, and press slightly. If using a cake ring, refrigerate the cake for about 30 minutes to stabilize the cream. Otherwise, proceed. Cover the cake with chocolate cream. Apply a fairly thin layer of the cream to the sides of the cake and sprinkle with the prepared crumbs.

To decorate the cake, transfer the chocolate cream to a piping bag with a Wilton open star tip M1. Pipe the cream around the edge of the cake. Transfer the white cream to a pastry bag with the tip M1.

Pipe two to three cream swirls or other floral designs with chocolate or white cream. Sprinkle half the cake with crumbs. Refrigerate the cake for at least 4-6 hours, preferably overnight.