Melt the butter and let it cool off.
Stir eggs and sugar together in a bowl.
Stir flour, cocoa, vanilla sugar and salt together in a bowl, mix it then in the sugar-egg mixture.
Pour the melted butter and stir.
Distribute the dough into the baking mold which should have a size of approx 25 x 20 cm and be lubricated with butter.
Chop the hazelnuts roughly and sprinkle them over the cake.
Bake the cake by 175°C / 347°F for approx 30 minutes, it must not get too much, it must be a bit soft inside,
test it with a fork if necessary.