Preheat the fan-assisted oven to 170°C / 340°C or a regular oven to 180°C / 350°C. Place the rack in the middle of the oven. Grease three 20 cm (8-inch) round non-stick springform cake pans. Don't forget to grease the sides too.TipIf you only have one 20 cm (8-inch) round non-stick springform cake pan, you will need to bake the sponge cake separately three times.
make the cream
Add gelatine to a bowl filled with 40 ml (3 tbsp) water and set aside for 5 minutes. Add whipping cream and sugar to a saucepan and bring to a boil over high heat. Remove from the heat and add the gelatine. Using a whisk, combine the mixture until the gelatine dissolves. Add white chocolate to a large bowl and pour the boiling hot whipping cream mixture over it. Set aside for 5 minutes, then stir well using a spatula. Stir in the mascarpone to get a smooth mixture. Cover with cling film so that the cling film touches the cream, then place in the fridge for 4 - 6 hours, or overnight. After that time, the cream should have a cream cheese consistency.TipIf you're in a hurry, stir the cream a couple of times while it's resting in the fridge. It will cool sooner.Optionally, use gelatine sheets instead of powdered gelatine. In that case, use 6 gelatine sheets and follow the instruction on the package. Drain well before using.
Combine blackberries, sugar, lemon juice, lemon zest, cornstarch, and water in a saucepan. Stir with a spatula, and place over medium heat. Bring to a boil, then simmer for 10 minutes—press mixture through a fine-mesh strainer into a bowl. Cover the bowl with cling film. Make sure the cling film touches the marmalade to prevent the skin from forming. Set aside to cool to room temperature, then transfer the marmalade to a fridge until needed.
In a small bowl, stir to combine the flour, baking powder, and salt. In a large bowl, beat together the eggs, sugar, and vanilla paste. Mix for 5 - 8 minutes on high speed or until the mixture triples in size. In a separate small bowl, stir together the milk and sour cream. Gradually, in alternations, start adding the dry ingredients and sour cream mixture to the egg mixture. Stir with a spatula until just combined before adding the next alternation. Lastly, stir in the melted (cool) butter. Pour the sponge cake batter into the prepared baking pans. Place in the preheated oven on the middle rack. Bake for 22 - 25 minutes at 170°C / 340°C (fan-assisted oven) or at 180°C / 350°C (regular oven). Remove the baked sponge cake from the oven. Place in the pan for a couple of minutes, then transfer to a wire rack to cool.TipBake all three sponge cakes at once. Place two pans in the middle of the oven, and place the third pan to the bottom of the oven. Once the first two (at the center) are baked, remove them from the oven and transfer the bottom pan to the middle of the oven. Continue baking for another 3 minutes or until the sponge cake is baked.If the top of the cake isn't even, use a serrated knife to cut off the top to make it even.
Remove the cream from the fridge and divide it in half. Stir Nutella into one half of the cream and blackberry marmalade into the other half. Beat both creams separately with an electric mixer for about 2 - 3 minutes or until they are creamy and smooth.
Assemble the cake
Transfer the sponge cake to a wire rack. Optionally brush each sponge cake with a tablespoon of syrup or liqueur. Spread the blackberry cream over the sponge cake, then place another sponge cake on top. Spread the Nutella cream over the sponge cake, and finish with the last layer of sponge cake. Using your hand, gently press the cake in the center toward the bottom to help the cream divide evenly. Using a spatula or a bench scraper, smooth out the cream on the sides of the cake. Place the cake in the fridge for 30 - 60 minutes.
decorate the cake
Pour whipping cream into a small saucepan. Place over medium heat and bring to a boil. Add chocolate to a bowl. Pour the boiling hot whipping cream over the chocolate in a bowl. Set aside for 5 minutes, then add the butter cut into cubes. Combine with a whisk. Remove the cake from the fridge and pour chocolate over the cake, starting in the center of the cake. Transfer to a serving platter and keep refrigerated until serving. Optionally decorate with fresh blackberries. Thirty minutes before serving, place the cake at room temperature.