Crumble the biscuits into crumbs using a blender. Melt the butter in a microwave and add it to the biscuits. Next, add the chopped walnuts. Knead everything and lay out along the bottom of a cake form with removable sides.
Mash the bananas, melt the chocolate in a water bath. Prepare the cream mixture by beating the cold milk and powdered cream with a mixer in a bowl. Mix at low speed at first, then turn it up to high for about 3 min.
Next, add the melted chocolate and mashed bananas. Stir everything well and add the 2 packets of gelatin, which you've dissolved beforehand in a little liquid, as instructed on the packets.
Pour the cream mixture over the biscuit layer.
Put the cake in the fridge to set for at least 4 hours. Remove the walls and decorate with spirals of white chocolate and banana pieces.