In a small saucepan, melt the butter over medium heat. Let it simmer, stirring constantly to ensure even cooking and prevent the brown bits from catching at the bottom. As you stir, you should see little brown flecks coming up into the pan, and the butter will turn a golden color.
Pour the browned butter into a clean bowl and let it cool for 2 minutes. Then, add the chopped white chocolate and stir until melted and smooth. Set aside.
Using a hand mixer, whisk together the eggs, both sugars, and vanilla until pale and thick, doubling in volume.
Mix in the salt followed by the melted white chocolate mixture.
Gently fold in the flour until combined. Then, add the dark chocolate chips and roasted hazelnuts.
Spread the batter evenly into the prepared pan. Sprinkle extra chocolate chips and hazelnuts on top.
Bake for 25 minutes until the top is shiny, golden, and crackly, and the center is still soft. Avoid overbaking.
Transfer the pan to a wire rack and let the blondies cool completely before slicing.
