For the brownies, melt butter and chocolate in a saucepan while stirring and let cool.
Beat eggs, vanilla sugar, salt and sugar until creamy. Add the chocolate-butter mixture and continue to stir briefly.
Fold in the chopped chocolate and walnuts, finally fold in the custard powder with baking powder.
Pour the batter into a well-greased baking tray and bake in a preheated oven at 160 degrees convection oven for about 20 – 25 minutes.
Cool the brownies and decorate with chocolate icing as desired.