Line a brownie pan with parchment paper. Spread ½ the jar of nutella in the parchment lined pan. Put in the freezer for 8 hours or until frozen. Once frozen, take out of the freezer. Take parchment lined nutella out of the pan.
Line the pan again with some parchment
Preheat oven to 350 degrees
Make brownie batter according to the box. Pour half the batter in the parchment lined pan.
Take the nutella slab off the parchment and place it on top of the batter.
Pour the rest of the brownie batter on top.
Bake for the amount of time recommended on the brownie box for the size of your pan (approx 30-40 mins)