Blend the biscuits in a blender until they turn to crumbs. Melt the butter in the microwave and mix it with the cocoa biscuits.
Spread out the resulting mixture in a cake form with removable sides and press firmly toward the bottom.
Whip the liquid cream to stiff snow with a mixer. Melt the chocolate in a water bath and add them to the whipped cream.
Add the sour cream as well, stir everything until homogeneous, so there's no lumps. Dissolve the gelatin as described on the packets in a little cold water or milk.
Wait a few minutes and put it in a water bath to liquefy. Wait for it to cool a bit and add to the creamy mixture. Stir the whole mixture. Pour it over the biscuit layer and smooth it out.