Preheat the oven to 325℉. Line an 8x8 baking pan with parchment paper and set aside.
In a medium bowl, mix together the flour and salt.
In a separate bowl, beat together the Nutella, brown sugar, vanilla and eggs until smooth.
Alternate between adding the flour mixture and the butter into the Nutella mixture. Mix until fully combined.
Pour the batter into the prepared baking pan and bake in the preheated oven for 50-55 minutes, or until a toothpick inserted comes out with only a couple of crumbs.
Remove from the oven and allow to cool for around 1 hour.
Cut into 16 brownies. Frost each brownie with around 2 Tablespoons of frosting and top with a marshmallow half. Decorate with black food writing gel, sprinkles and a mini peanut butter cup.