Preheat your oven to 350°F (175°C) and line an 8×8 inch baking pan with parchment paper, leaving some overhang for easy removal later.
In a large bowl, whisk together the Nutella, melted butter, and sugar until completely smooth and well incorporated. This mixture should look glossy and have no sugar granules visible.
Add eggs one at a time, whisking well after each addition until the mixture becomes slightly lighter in color and develops a silky texture.
Stir in the vanilla extract until fully incorporated into the chocolate mixture.
Gently fold in the flour and salt with a spatula, being careful not to overmix. Stop stirring as soon as no dry flour is visible to ensure tender brownies.
If using, fold in chocolate chips and/or chopped hazelnuts, distributing them evenly throughout the batter.
Pour the batter into your prepared pan and smooth the top with a spatula for even baking.
Bake for 25-30 minutes until the edges are set and a toothpick inserted in the center comes out with a few moist crumbs (not wet batter).
Allow the brownies to cool completely in the pan on a wire rack before cutting to achieve clean slices and let the flavors develop.
Once cooled, lift the brownies out using the parchment paper and cut into 16 squares.