Preheat your oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper, allowing some overhang for easy removal later.
In a large mixing bowl, combine the melted butter and Nutella. Whisk together until smooth and creamy.
Add in the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the granulated sugar, flour, salt, and baking powder until evenly combined.
Gradually add the dry ingredients to the Nutella mixture, stirring gently until just combined. If desired, fold in chocolate chips for extra gooeyness.
Pour the brownie batter into the prepared baking pan, spreading it out evenly.
For a beautiful swirl effect, drop spoonfuls of additional Nutella on top of the brownies and use a knife or skewer to swirl it into the batter.
Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs (you don't want to overbake them).
Allow the brownies to cool in the pan for about 15 minutes before using the parchment overhang to lift them out. Let them cool completely on a wire rack before slicing into squares.
Notes
Serve with powdered sugar and vanilla ice cream for a delightful experience.