Preheat oven to 350°F and line 2 sides of an 8x8 pan with parchment paper. Spray the other two sides with oil. Set aside.
Melt the butter in the microwave or on the stovetop. Allow to cool for 10 minutes. Then in a large bowl, whisk brown sugar, white sugar, and butter.
1/3 cup butter, 1/3 cup brown sugar, 1/4 cup white sugar
Add the eggs and vanilla and continue whisking. Add in 1 cup of Nutella and fold in using a spatula.
1 1/2 cups Nutella, 2 eggs, 1 tbsp vanilla
Fold in flour and unsweetened cocoa powder.
2/3 cups all-purpose flour, 3 tbsp unsweetened cocoa powder
Pour the brownie batter into the prepared pan and spread it evenly. Drop teaspoons of the remaining ½ cup of Nutella over the top of the batter. Use a knife to swirl the Nutella into the batter.
Bake for 30-35 minutes, or until a toothpick comes out with a few moist crumbs. The edges will be crisp and the middle will be fudgy. Top with flakey sea salt.
Let the brownies cool, slice into squares and enjoy!