To make Nutella brownies, preheat an oven to 375 degrees Fahrenheit. Set a saucepan over medium heat, and add 1/2 cup of butter, 1/4 cup of hazelnut liqueur and 1/4 cup of Nutella. Heat and stir the ingredients until they bubble, approximately five minutes, and then remove the pan from the stove to cool.
In a bowl, combine 1/2 cup of unsweetened cocoa powder, 1/4 teaspoon of salt and 1 cup of flour. In a separate bowl, add 2/3 cup of white sugar, 1/3 cup of brown sugar, 2 eggs and 1 teaspoon of vanilla extract. With an electric or hand mixer, whisk the wet ingredients together until they are smooth and thoroughly incorporated. Mix in the Nutella and butter mixture to the wet ingredients, and then add in the dry ingredients. Mix the batter just enough to thoroughly combine all the ingredients.
For more chocolate flavor, fold in 1/2 cup of semisweet chocolate chips. Prepare an 8-inch square pan by lining it with parchment paper or greasing it. Transfer the batter to the pan.
Bake the brownie batter for 20 to 25 minutes. When the brownies are fully cooked, the top feels dry to the touch and the edges of the brownies begin to separate from the sides of the pan. Allow the brownies to cool completely before serving.