Preheat your oven to 350 degrees.
Add your hazelnuts to a baking sheet. Toast them in the oven for 7 minutes, being careful not to burn them. Remove from the oven and allow to cool. Once cool, pour all hazelnuts into a clean dishtowel. Rub the hazelnuts around inside the dish towel to remove the skins. Roughly chop and set aside.
In a large bowl, combine the melted butter, brown sugar and ¾ C of the chocolate hazelnut spread. Mix together until well combined.
Add the eggs one at a time. Beat on medium speed for about 30 seconds between each addition.
Add the vanilla. Scrape down your bowl.
Add the flour, cocoa powder and salt to the wet ingredients. Mix on low speed until just combined.
Add ½ C of the chopped hazelnuts to your batter. Fold together with a spatula.
Line an 8x8 pan with parchment paper.
Pour your brownie batter into the pan, making sure to spread it evenly across the pan.
Dollop the remaining ¼ C of chocolate hazelnut spread on top of the brownies with a teaspoon. Once you have all the dollops, run a knife through the batter to make a swirl.
Sprinkle the remaining chopped hazelnuts on top of the brownies.
Bake for 30 minutes in preheated oven (27-28 minutes if you want them to be gooey).
Remove from oven and cool for 1 hour before cutting.