Melt butter, sugar and cocoa powder in a water bath and mix well. Add the egg and heat while stirring until the mixture starts to thicken. Remove the pot from the heat and then stir in the remaining ingredients. Finally, place the mixture in an ungreased baking pan (20 cm x 20 cm) and press well.
Mix all ingredients into a slightly creamy mass. Then spread the mixture onto the floor.
Melt chocolate and butter over low heat, mix and let cool down a bit. Apply the slightly cooled but still liquid mass to the middle layer. Then put the whole thing in the fridge to cool down.