Put the 2 tbsp cocoa in 4/5 cup (200 ml) water to come to a boil. Beat the butter with a mixer in a container, then add the powdered sugar.
Once the cocoa cools, add it to the butter while stirring nonstop. Put the sugar in a pan and heat it on the stove until it caramelizes. Pour it into foil.
Wait for it to cool and then put it in a blender to break it up. Put all of the caramelized sugar in a bowl, where you've also put the ground walnuts.
Cut the ready-made cake layers into cubes. Pour them in a deep container and add the buttered mixture to them, while stirring very carefully.
Pour the mixture into a form with removable sides, smooth it out on top and sprinkle with the walnuts with caramel.
Put it in the fridge for 24 hours before serving.