Preheat the oven to 350°F (175°C). Grease and line a 9×9 inch baking pan with parchment paper.
In a saucepan, melt the butter over low heat. Remove from heat and stir in the cocoa powder until smooth.
In a mixing bowl, beat the eggs and sugar until light and fluffy. Add the vanilla extract and salt.
Gradually pour in the cocoa-butter mixture while mixing continuously.
Fold in the flour and chopped hazelnuts until just combined.
Pour the batter into the prepared pan and spread it evenly.
Bake in the preheated oven for 30 minutes or until a toothpick inserted in the center comes out with moist crumbs.
Allow the brownies to cool in the pan before slicing into squares.
Serve and enjoy these Hazelnut Heaven Brownie Slices!