Put butter, sugar, vanilla sugar, salt, Nutella and 1 mashed banana in a large bowl and beat with the mixer until soft and creamy. Gradually add the eggs. Sift the flour with the cocoa powder into the bowl, add the nuts and fold in.
Spread on a greased tray (20 x 20 cm) lined with baking paper. Slice the other banana and spread it over the brownie mixture.
Bake in the oven preheated to 180 ° for about 25 - 45 minutes. Depending on the baking pan used!