Preheat the oven to 350°. Line a 9×9 baking pan with aluminum foil. Spray foil inside with some cooking oil spray.
In a small sauce pot, melt butter over medium heat and whisk in 1 cup of Nutella until all combined and smooth. Take off heat and let it cool for a couple of minutes.
Pour Nutella mixture into a large mixing bowl and whisk in sugar.
Make sure that the mixture is not too hot and add eggs, one at a time, whisking well after each addition. Whisk vigorously for about a minute.
Whisk in the vanilla extract, flour, baking powder, and salt until smooth.
Transfer the mixture into the lined baking pan and spread it evenly.
Warm up remaining 1/2 cup of Nutella and spoon a dollop of Nutella all over the top of brownie mixture. Using a thin spatula, swirl Nutella spread all over the top of the brownie batter in figure 8s. Gently shake the pan a couple of times to make sure batter is all even.