Preheat oven to 350 degrees F. Spray an 8-inch by 8-inch square baking pan with a non-stick cooking spray or line the pan with foil, leaving an overhang on the sides, and spray the foil with a non-stick spray.
In a large bowl, using a stand mixer (fitted with a paddle attachment) or electric hand mixer, beat the butter until smooth and creamy. Add the brown sugar; mix until combined.
Add in the eggs; mix until combined. Set aside.
In separate bowl, whisk together the salt and flour.
Pour the flour mixture into the butter and sugar mixture, pour the Nutella in, and mix everything until well combined. Set aside.
Cut the chocolate hazelnut candies into quarters. The cream inside will make them delicate, take care not to smash them. Set aside.
Spoon the brownie batter into the prepared pan and spread to the edges.
Pour the ¼ cup Nutella topping in lines across the brownie batter. Using a skewer, toothpick, or knife, create a swirling pattern through the Nutella. Top the brownies with the candies and lightly push them into the batter so they stick.
Bake the brownies for 30-35 minutes, until the edges start to brown and a toothpick inserted into the center comes out clean.
Cool completely in the pan on a wire rack before removing from pan, allowing the candies to reharden.
Store covered in an airtight container at room temperature or they can be refrigerated for up to 4-5 days. These will remain good for several days, even up to a week.