Beat all the ingredients for the coffee cream, except for the biscuits, blackberries, and gelatin.
Hydrate the gelatin and microwave for a few seconds, then add it to the coffee mixture and beat. Set aside.
Put a base of biscuits in the rectangular baking pan and pour some of the coffee mix, then add the blackberries. Repeat until filling all the baking pan, finishing with the coffee mixture. Refrigerate for 4 hours.
For decoration, make a ganache, heat the cream until it boils. Pour over the chocolate and mix well. Allow to cool.
Remove the cake from the mold and bathe with the chocolate ganache, blackberries, and walnuts. Serve.