Preheat the oven to 180 °C (350 °F). Beat the eggs and sugar at maximum speed for 10 minutes. Sift the cocoa and flour on top of the egg mixture and gently fold in with a spatula. Make sure that the mixture remains airy. Spread the mixture evenly on an 18 cm cake pan lined with parchment paper (the bottom). Bake for 15 minutes. While the sponge is in the oven, prepare the blueberry layer and the sugar solution.
In a small saucepan, bring the blueberries, water and sugar to a boil. Soak the gelatin in cold water for 5 minutes. Take the cooked strawberries off the stove, add the wrung-out sheets of gelatin and stir to dissolve the gelatin. Line a 14 cm (4 cm smaller than the diameter of the cake) cake pan with plastic wrap, securing the wrap by wrapping it tightly around the pan. Pour the cooked blueberries into the cake pan and place in the fre- ezer for the layer to set. Cool the sponge in the baking tin for 5 minutes, then take it out and cool on a cooling rack.
While the sponge cools, prepare the blueberry filling. In a small sauce pan, bring water and sugar to a boil. Soak gelatin sheets in cold water for 5 minutes. Whip the cream and refrigerate. Blend the cooked blueberries with an immersion blender and dissolve the wrung-out sheets of gelatin in the warm (not hot) purée. Mix the mascarpone cheese and powdered sugar until smooth using a hand mixer; add the blueberry purée and mix well to combine the ingredients. Finally, fold in the whipped cream using a spatula.
Cut the sponge cake into two equal layers. Cut the top layer of the sponge to a 14 cm circle with the help of the cake ring or a cup. Put the bottom layer of the sponge on a stand (or cake pan), moisten it with the sugar solution and fit the cake ring. Spread the blueberry filling on the sponge and level out the surface. Put the cake in the refrigerator to allow the filling to set, in the meantime, prepare the white chocolate filling.
Melt the chocolate with 50 g whipping cream in a double boiler; whip 200 g whipping cream to medium stiff peaks. Soak gelatin sheets in cold water for 5 minutes. Dissolve the wrung-out sheets of gelatin in the melted chocolate and cool until lukewarm. Fold whipped cream into the mixture with a spatula.
Place the smaller sponge in the middle of the blueberry filling and moisten with sugar solution. Spread two tablespoons of filling on the sponge and place the blueberry layer on top. Coat with the rest of the filling (fill the space around the sponge first) and level out the surface. Place the cake in the refrigerator to allow the filling to cool. Pour the blueberry jelly over the cooled filling. To prepare the jelly, bring the blueberries, water and sugar to a boil; soak the sheets of gelatin in cold water for 5 minutes. Blend the hot blueberries using an immersion blender, allow them to them cool slightly and use to dissolve the wrung-out gelatin. Let the cake cool for at least 4 hours. Remove the rim from the chilled cake and decorate as desired.