Preheat the oven to 350 F. Line an 8x8 baking pan with parchment paper and coat with baking spray.
In the bowl of a stand mixer fitted with a paddle attachment, mix the butter and brown sugar on medium speed for 2 minutes. With the mixer still on medium, add the eggs and vanilla and continue mixing until smooth, scraping the sides of the bowl as necessary.
Turn the speed to the medium-low and add 1 cup of the Nutella and the salt mixing until smooth and fluffy.
Turn the speed to low and add the flour, mixing until just combined.
Spread the batter evenly into the prepared pan. Drop the remaining 1/4 cup Nutella by the teaspoon evenly over the batter, then swirl the Nutella into the batter, using a butter knife.
Bake for 30 to 35 minutes until the center is just set.
Allow to cool completely in the pan on a wire rack. Cut into squares when ready to serve.