When sandwiched in between brownie batter, Nutella transforms into a gooey, melty delight that takes traditional brownies to a whole new level of deliciousness.
Line an 8x8 square baking pan with parchment paper, making the sides a little longer than the sides of the pan to use as handles.
Spread the jar of Nutella in a thin layer to coat the bottom of the parchment paper inside the pan. Place the pan in the freezer to freeze Nutella for 1 hour or until frozen solid.
Once Nutella is frozen remove the frozen Nutella from the pan so pan can come back to room temperature. Keep Nutella in freezer while mixing brownie batter.
Preheat oven to 325 degrees.
In a small saucepan, melt butter. Add the semi-sweet chocolate to the melted butter and stir to melt. Remove from heat and whisk in vanilla extract and vegetable oil.
In a large bowl, sift the powdered sugar. Add the eggs and egg yolk and mix with a hand mixer or whisk until combined. Be careful not to over mix.
Stir the cooled chocolate into the egg mixture.
Combine the flour, cornstarch and salt. Add the flour mixture to the chocolate. Use a spatula to combine the ingredients, being cautious not to over mix. Stir in chocolate chips.
Pour half of the brownie batter into the parchment lined baking pan.
Place the frozen Nutella on top of the brownie batter. Spread the remaining brownie batter over the Nutella.
Bake in preheated oven for 35 minutes. Brownies are done baking when the edges are pulling away from the sides and center is set. (You can not check for doneness with a toothpick because of the Nutella center.)
Allow to cool in pan for 30 minutes. Use parchment to carefully lift brownies out of pan and onto a wire cooling rack to cool for 1 additional hour before cutting. Brownies cut better when they have completely cooled.