Preheat the oven to 180°C (350°F) and line an 8"x8" baking pan with baking paper and set aside.
Beat the partially melted butter, brown sugar and sugar together.
Beat in the vanilla extract and egg.
Beat in the flour and baking soda until all combined.
Stir through the chocolate chips.
Press half of the mixture into the base of the baking pan.
Spread the Nutella over the top in an even layer.
Pour over the condensed milk (if using).
Place over the marshmallows in a spread out formation as they will spread as they cook.
Press over the rest of the dough in an even layer covering the Nutella and marshmallows.
Bake for around 20 minutes. The top and sides with be golden and the middle will wobble slightly but not too much. When a skewer entered in the middle comes out with a few light crumbs attached it is cooked.
Don't add the condensed milk if you don't want this overly gooey.