Grease a 30 x 20 x 5 cm baking sheet and line it with parchment paper.
Beat the butter with the light brown sugar until frothy. Gradually stir in the eggs and vanilla flavor. Sift flour, baking powder, salt and cocoa onto the dough and fold in. Fold in toasted hazelnuts, chocolate and marshmallows. Pour the dough onto the prepared tray.
Bake in the preheated oven at 180 ° C top / bottom heat for approx. 30 - 35 minutes. Let cool in the tray and only then cut into 16 pieces.