1. Preheat the oven to 180°C. Grease and line a skillet pan.
2. In a stand mixer, beat the butter and sugar until light and fluffy, about 2 minutes.
3. Add the eggs and beat again. At this point it may look like the eggs have scrambled the butter mixture but don't worry. It will all come together once the dry ingredients have been added.
4. Add in a generous amount of Nutella and continue mixing.
5. Next, add the flour, cocoa powder, baking powder and salt. Turn the mixer on low.
6. Beat for 1 minute until smooth.
7. Pour the brownie batter into the skillet and spread evenly.
8. Bake for 25 minutes. The brownies will have a slight wobble. Sprinkle over some chocolate chips and place your marshmallows on top.
9. Bake again until the marshmallows turn golden brown.
10. Leave to cool slightly and dig in!