Start with the prunes, wash them and pour boiling water over them. Once softened, drain and cut them into pieces. Put them on a piece of paper towel to drain further.
For the cream mixture, mix the yoghurt with the sour cream and sugar, stir well. Beat the cream with a mixer into fluffy cream, add the yoghurt with sour cream mixture to it in lots and stir - you have to obtain a thick, fluffy cream.
For the layers, separate the egg whites from the yolks, beat the whites with the sugar into cream, add the yolks, milk, vanilla sugar and cocoa, stir until homogeneous.
Add the premixed together sifted flour and baking powder in stages, while stirring nonstop with a spatula to obtain a smooth mixture.
Smear an 11″ (28 cm) diameter tray with butter, pour the mixture in it and put the tray in a preheated 392°F (200 °C) oven to bake until ready. Check with a toothpick to see if it is. Take the tray out and leave it for 10 min. for the layer to cool.
Take the layer out of the tray, cut it into 2 equal parts horizontally and leave to cool.
In the meantime, prepare the syrup for the layers. In a metal container, melt the sugar into a caramel, add the water and once boiling, add the cognac. After 2-3 min. of boiling add the vanilla sugar and remove from the heat.
Start assembling the cake. Put 1 of the layers in a platter and pour on 1/2 the syrup. Mentally, divide up the cream mixture into 3 parts, spread on 1 part of it evenly, arrange the pieces of prunes on top, place the 2nd part of cream mixture and even it out.
Place the 2nd layer, pour on the rest of the syrup. Distribute the 3rd and final part of cream mixture on the top and sides of the cake, even it out very well.
Make the chocolate glazes: pour the cream into a metal container, crumble the dark chocolate into it and heat until it melts to a smooth mixture. Pour it over the cake on top and even it out. Prepare the white chocolate in the same manner.
Place clumps of white glaze here and there over the dark glaze. Use a toothpick to smear the white chocolate clumps and spread them out into snowflakes. Stick the ground walnuts on the sides of the cake.
A ridiculously delicious and aromatic cake for New Year's Eve!