Line square baking pan with parchment paper.
Spread full jar (or ~8 oz.) Nutella in thin layer over parchment paper in baking pan.
Place in freezer for at least 1 hour until Nutella forms a solid layer.
Remove parchment paper with frozen Nutella from baking dish. Place back in freezer to
keep firm while preparing brownie batter.
Preheat oven to 350 degrees Farenheit. Melt butter and chopped baking bar in microwave in 30 second increments until full melted
and smooth. Stir after every 30 seconds. Mixture should be fully melted within 1.5-2
minutes.
Add oil, granulated sugar, brown sugar to butter/chocolate mixture and stir.
Add in eggs + egg yolk and vanilla extract. Mix in flour.
Fold in chocolate chips.
Grease baking dish (be sure baking dish is at room temperature. Do not put dish straight
from the freezer into the oven.)
Spread half of the brownie batter into the greased baking dish.
Remove Nutella layer from freezer and peel away parchment paper. Place Nutella sheet
over the first layer of brownie batter.
Top with remaining brownie batter.
Bake for 30 minutes.
Remove from oven and allow to cool completely before cutting.