Put the gelatin in a container with water until it swells. Melt it in a water bath while stirring nonstop. Leave it to cool at moderate temperature.
Beat the sour cream well with the powdered sugar. Take the fruits out from the first compote, drain them. Use the juice to syrup the cake layer.
Pour the 2nd compote into the cream and beat again. Then add the gelatin to the mixture and beat with a mixer.
In a cake form, arrange in this order: cake layer, jam, walnuts, mixture. Put another thin layer of compote fruits or jam on top if desired. Put it in the fridge to harden.