How to cook
For the layers, beat the egg yolks with the sugar to a smooth cream in a container in a water bath on the stove. Be sure to beat on low heat and once the mixture becomes a fluffy cream, remove it and whip continuously for 3 more minutes. Add the cooled melted butter and sifted flour, stir with a spoon just long enough to homogenize it and then stop stirring to prevent the mixture from deflating.
Then carefully mix with the beaten egg whites. The spongy layer can also be made by beating the eggs whole without separating the eggs from the whites. Pour half of the mixture into a form with a diameter of 8″ (20 cm), laid out with baking paper. Bake the 2 layers separately, one after the other in a preheated 356°F (180 °C) oven. Let the cake layers cool completely and cut them in 2, to get a total of 4 layers.
For the cream for the cake, beat the sour cream very well. Heat the liquid chocolate a little and add the gelatin that's been dissolved in a little water to it. Stir quickly and allow the mixture to cool. Then carefully mix it with the beaten cream, stirring with a spatula. Finally, gently mix in the crushed hazelnuts.
For the buttercream topping, beat the soft butter with the powdered sugar, a pinch of salt, the cocoa, vanilla, liqueur, milk and the slightly liquid, but still unheated chocolate. If necessary you can add a little more milk.
Stick the four layers together in a suitable tray with chocolate cream (optionally, syrup them very slightly with milk or sugar). Coat the cake on top with the buttercream and smooth it out well. Put the cake to cool while preparing the sugar dough for the decorative roses.
For the sugar dough ornaments, dissolve the gelatin in a little water for 10-15 minutes. Melt it briefly in a dish on the stove and add the sifted powdered sugar into it. Add just a little lemon juice. Initially, stir it, then knead with your hands. If it is runny, add more powdered sugar. Kneading takes about 15 minutes, until you get an elastic mass, similar to plasticine.
Optionally, you can color the sugar dough with confectionery dye. Tear off small pieces from the dough, rolling them out until smooth and shaping them into rose petals, placing them within each other. Place the resulting roses at one end of the cake, making a few of them in different sizes.
Color a portion of the dough with green dye and form the rose leaves. Roll out the dough thinly again, cut out leaf shapes, make cuts with a knife carefully to make them look like leaves, and gently wrap them around the roses.