Preheat the oven to 355°F/180°C. Line a baking pan (6 x 9 inches | 15 x 23cm) with parchment paper.
Whisk soft butter and brown sugar in the bowl of a stand mixer (a handheld mixer works too) until creamy. Add eggs and ½ cup of Nutella (keep ¼ cup on the side for the topping) and mix until just combined. Fold in flour and salt and pour the batter into the prepared baking pan.
Drop teaspoonfuls of the remaining ¼ cup of Nutella on top of the batter. Using a spoon or knife, carefully swirl the Nutella into the batter - don't overmix if you like a creamy top!
Bake the brownies for 25 minutes in the preheated oven. Remove from the oven, place on a wire rack, and let it cool completely in the baking dish.
Cut into 12 brownies & enjoy!
Notes
Sugar: Those brownies are made with less sugar than regular brownies, thus they are less sweet. Feel free to add ¼ cup white sugar for sweeter brownies!
Store leftover brownies in an airtight container at room temperature or in the refrigerator for up to 2 days.