Preheat oven to 350°F. Line an 8"x8" square pan with a parchment paper sling.
In a large bowl, whisk the butter, brown sugar, and granulated sugar until combined. Add the eggs, vanilla, and salt, whisking again until combined.
1/3 cup salted butter, 1/3 cup brown sugar, 1/2 cup granulated sugar, 2 large eggs, 2 teaspoons vanilla extract, 1/2 teaspoon salt
Whisk in 1 cup of the nutella until combined.
1 1/2 cups Nutella
Sprinkle the flour, cocoa powder, chocolate chips, and hazelnuts, if using, over the batter, then stir just until combined.
2/3 cup all-purpose flour, 2 Tablespoons cocoa powder, 2/3 cup semisweet chocolate chips, 1 cup chopped hazelnuts
Pour the brownie batter into the prepared pan, spreading it evenly into the corners. Drop teaspoonfuls of the remaining 1/2 cup of nutella over the top of the batter. Use a knife to swirl the Nutella into the batter, then sprinkle with flaky salt, if desired.
Flaky sea salt for sprinkling
Bake for 30-35 minutes, just until a toothpick comes out clean with only a few crumbs when used to test the brownies.
Let the brownies cool completely before slicing into squares.