Preheat the oven to 350°F. Grease and line an 8-by-8-in baking pan with parchment paper.
In a medium-sized bowl, whisk together the all-purpose flour, cocoa powder, and salt.
In a large bowl, whisk the melted butter and sugar together until incorporated. (Whisk well for 1-2 minutes.) Add the eggs and vanilla extract, whisk for an additional 2 minutes, until light in color. Carefully whisk in the ¾ cup of Nutella (we do not want to deflate the whipped egg mixture).
Finally, gently fold the dry ingredients into the Nutella-egg mixture. Transfer the batter to the prepared pan and smooth it into an even layer.
Drop spoonfuls of the remaining Nutella on the surface of the brownie batter. Using a knife, swirl the Nutella into the batter.
Bake for about 20-25 minutes, or until a toothpick inserted into the center comes out clean. Allow the brownies to cool before cutting them into slices.
Store the brownies in an airtight container at room temperature for up to 4 days, or in the refrigerator for up to one week. Freeze for up to 2 months.