Crumble the biscuits into small pieces. In a water bath, melt the butter and stir in the cocoa, sugar and rum. Stir to a smooth cream. Sprinkle the biscuits in the dish with the milk and stir them with the walnuts.
Pour all the cream on top and stir gently. Pour the mixture into a small rectangular form and smooth out the top. Melt 1/3 cup (80 g) butter, remove from the heat and mix in the powdered sugar.
Brush the top of cake with a thin film of this sugar cream. Sprinkle with grated chocolate and place in the refrigerator for several hours.
Optionally, make decorations. Leave the cake for 10 minutes at room temperature before serving.