Line the 26-cm (10.2 inches) spring-form cake tin with removable base with a parchment paper and divide the eggs into two separate bowls. Preheat the oven to 175 ° C (347 °F), with both top and bottom heat. Add salt to the bowl with egg whites and beat until light and fluffy. Next add granulated sugar and vanilla sugar to the bowl with egg yolks and beat until creamy. Mix hazelnuts, plain flour (all purpose flour) and baking powder and add to the egg yolk mixture and mix well. Then fold gently the egg whites into the egg yolk – flour mixture. Pour the batter into the prepared spring - form cake tin lined with a parchment paper and bake for 40 minutes or until nicely golden brown. Remove the cake from oven and let cool completely (preferably overnight).
Once the sponge cake has cooled completely make the cherry filling. Simply drain the cherries, keeping the cherry water. In a pot whisk sugar and cherry water from the drained cherries, leaving ¼ behind and bring to a boil. In the meantime mix cornstarch with cherry water that you kept aside and add to the pot once the water starts boiling. Keep stirring constantly until it starts to bubble and has thickened. Remove from the heat and stir in drained cherries and bitter almond oil. Let cool for few minutes while cutting the cake into layers.
To assemble the marzipan torte, remove the sponge cake from the cake tin and put it on a cake board/ platter. Using a long knife make marks on the cake, such that in the end you'll have 3 layers. While holding the knife with the right hand, place your left hand on the cake to hold it, while lightly applying pressure and and cut it horizontally. Cut the cake layers starting from bottom to top. You could also use a thread to cut it. Using a cake lifter or a thin plastic cutting board lift up the first two layers and put them aside.
Now take a cake ring or the ring from the spring-form cake tin that you've used to bake the cake and put it around the bottom sponge cake layer with the cut side facing up. Spread the cherry cream/ pudding (that you just made and cooled for few minutes but still hot ) on top. Cover with cake keeper and chill until the cherry cream has cooled completely. This takes about 40 minutes in the fridge. In the meantime make the whipped cream and hazelnut filling and roll out the marzipan.
Dust a pastry mat or cutting board with powdered sugar and thinly roll out marzipan into a round form. Now take a cake ring and place it on the rolled out marzipan. Then cut off excess marzipan. (See photos). You just want to make sure that in the end you have a well formed circular shaped marzipan. (You could also use a ruler or measuring tape to check the size of the marzipan, if you're already using the spring-form cake ring around the cake).
Put about 6 tablespoons ground hazelnuts in a pan without oil and roast them for about 5 minutes. Roasting gives the nuts extra fine nutty flavor. Next, pour the very well chilled whipping cream in an empty jar or bowl (chilled) and whip with well chilled beaters on low speed. Add vanilla sugar and bitter almond oil. Increase the speed and whip until light and fluffy. Using a spatula fold in roasted nuts, gently. Now take out the bottom sponge layer with cherry pudding cream from the fridge. Scoop about 1/3 of the whipped cream on top the cherries and smooth with spoon or spatula. It's very important that the cherry pudding has cooled completely before adding cream on top. Take the rolled out marzipan and place it nicely on top of the cream layer. Then cover with the middle sponge cake layer. Sprinkle the top of the middle sponge layer with cherry juice and spread with the remaining whipped - nut cream. Top the nut - whipped cream layer with the last layer of sponge cake, with cut side down. Cover with cake keeper and chill for at least 3 hours.
Shortly before taking the cake out of the fridge roll out the marzipan for décor, just like the way you rolled out marzipan for filling. Place a round cake cutter/ divider on top of the marzipan that you've prepared and lightly apply pressure to mark the slices. Remove the cake cutter and cut into 16 slices. (You could also use a ruler and a knife to make marks on the marzipan before cutting. Just think about a round clock.). Now place the marzipan potatoes at the middle of every marzipan slice and roll up from the middle as shown on the photos. Keep dusting the working surface with powdered sugar so that marzipan won't be sticking.
Next whip the whipping cream for décor and roast chopped hazelnuts. Now take a knife and coat the sides and top of the cake with a layer of whipped cream, leaving about ½ of the cream behind for decorating the top of the cake. Use the half lemon to sprinkle chopped hazelnuts on the sides of the cake. (Using a lemon makes it easier to sprinkle the nuts. Besides you won't be having lots of nuts lying around the working surface) Simply dip the lemon in roasted hazelnuts and then on the cake. If there is cream on the lemon, just wash it and dry with paper towels and repeat the process, until all the sides are coated with hazelnuts.
Using a knife or the same cake divider that you've used to divide the rolled out marzipan, mark the slices of the cake. Place the rolled up marzipan slices on the cake (where you've marked with the cake divider). Scatter the remaining roasted nuts in the top center. Fill a pipping bag with the remaining whipped cream and pipe small swirls on top of the cake (on every marzipan piece).