Pre heat oven to 180c (350f).
Grease and line an 18cm/7" square (base measurement) cake tin and set aside.
In a medium sized bowl, combine the salted butter (75g) and the golden caster sugar (½ cup/100g) and using a hand held mixer, mix until fluffy.
Add eggs (2) and vanilla essence (2 tsps) mixing until combined.
Add Nutella (1 cup/300g) and mix well until combined.
Add plain flour (¾ cup/120g) and mix by hand until just combined. Don't overmix.
Fold in chocolate chips (⅓ cup) and peanut butter chips (⅓ cup), then pour batter into prepared tin.
In a small bowl, combine the peanut butter (2 tbsp) and the remaining Nutella (2 tbsp) then place dollops of this mixture evenly on top of the batter mixture. Take a knife and gently swirl the peanut butter mixture to combine slightly (see photos above) and bake for 34 mins (see notes).