Melt butter in a saucepan and let it cool slightly.
Whisk sugar, brown sugar, vanilla sugar, and eggs together until light and fluffy.
Mix flour, cocoa, and salt together and sift it into the batter.
Add the butter to the batter and gently fold it together until uniform.
Pour half of the batter into a small baking pan lined with parchment paper and distribute Oreos and half of the marshmallows on the batter.
Pour the remaining batter on top and smooth it out, so everything is covered.
Bake the cake at 180 degrees Celsius (356 degrees Fahrenheit) in a conventional oven for about 13-15 minutes.
Remove the cake and distribute the remaining marshmallows on the cake.
Bake the cake for an additional 10 minutes.
Let the cake cool - preferably in the fridge - for at least 2-3 hours so it is completely cold.
Cut and serve ??Enjoy the recipe!