WHAT TO DO:
1. Preheat oven to 180°C. Then spray the baking tin with cooking spray and line it with baking paper. We used a 20x23cm baking tin.
2. In a bowl, mix the butter and brown sugar until crumbly.
3. Add in the eggs and vanilla extract until the mixture is smooth.
4. Mix in the salt, baking powder and then flour. Your mixture should now be wet.
5. Spread the mixture into the tin - make sure to get it all the way to the edges and spread it nice and evenly! If the mixture is sticking to your hands, spray them with some oil and it will magically fall off!
6. Bake for 20-25 minutes or until slightly golden. Let it cool completely before touching.
7. When the slice has cooled, spread a layer of Nutella on top.
8. Add the marshmallows to the top. Do a couple of layers and cover the Nutella.
9. Put the slice back into the oven for about three minutes, but keep a close eye on it as it might burn quickly. When the marshmallows are looking golden, remove the tin from the oven and let it cool.
10. Cut the slice up and enjoy this sticky sweet treat!