Preheat oven to 350F/180°C. Grease an 8x8-inch/20x20cm square pan and line the bottom and the sides with parchment paper. Leave an overhang on every side to easily lift the brownies out of the pan. Set aside.
Using a hand mixer or stand mixer fitted with paddle attachment, cream together butter and brown sugar until well combined. Add eggs and vanilla extract and mix until combined. Then, beat in 1 cup (300 gr) Nutella until well incorporated.
In a large bowl, combine together all-purpose flour, cocoa powder, and salt. Add dry ingredients to wet ingredients and mix until incorporated.
Pour the batter into the prepared pan. Drop the remaining Nutella in small dollops on the top of the brownies batter. Using a toothpick or knife, swirl the batter and Nutella together in an up and down motion. Then, repeat the same motion crosswise until the desired swirling is achieved.
Bake for 35 minutes or until a toothpick inserted in the center comes out with a couple of moist crumbs. Cool completely in the pan before slicing.