Preheat oven to 350F (180C). Line the bottom of an 8-inch square baking pan with parchment paper, letting the paper hang over the sides. Set aside.
In a bowl combine flour, cocoa powder, salt, and baking powder.
In the bowl of a stand mixer or using a hand mixer, combine eggs, 1 cup Nutella, brown sugar, and vanilla extract. Beat for about 2 minutes until smooth.
With the mixer running on low speed, add the flour mixture alternating with the melted butter. Stir in 2/3 of the chopped hazelnuts.
Spread batter into the pan, then drop remaining Nutella by the tablespoon onto the batter and use a knife to swirl it into the batter. Sprinkle remaining hazelnuts over the brownies and slightly press them into the batter.
Bake in the middle of the oven for 30-35 minutes or until a toothpick inserted into the center comes out with only a few moist crumbs attached.
Set baking pan on a cooling rack and let brownies cool in the pan to room temperature. Then lift out the brownies using the parchment paper and transfer to a cutting board. Cut into squares.