Melt the dark chocolate in a water bath with the liquid chocolate. Add the biscuits, stir until you get a smooth mixture and distribute it along the bottom of a cake form. Smooth out the surface.
Make the cream by melting the butter and milk chocolate in a water bath, then add the milk and stir until smooth. In a bowl, beat the yolks with the sugar and combine with the chocolate mass.
Stir again until you get a smooth cream. To it, add the sour cream, which you've beaten in advance. Stir well and pour it over the cake layer. Sprinkle with hazelnuts and keep in the fridge until it's time to serve.