Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper leaving a 2-inch overhang along each side.

Brown the butter. Melt it over medium heat in a small saucepan. Make sure to use a stainless steel saucepan so you can see what's happening. If you use a dark pan it may be difficult to monitor the change in color. Once melted, let the butter come to a simmer. It will crackly as it boils. Stir the butter constantly as it melts to ensure it cooks evenly and so the brown bits don't catch at the bottom. As the moisture evaporates, the butter will start to foam and eventually the crackling will settle down and the foam on the surface will become very dense and it will turn golden. As you stir you should see little brown flecks coming up into the pan, the butter will turn a golden color and it will have a toasted, caramelized and nutty aroma.

Immediately pour the browned butter into a clean bowl and let it cool for 2 minutes so it is not boiling hot, then add the chopped white chocolate. Stir until the chocolate is completely melted and the mixture is smooth. Set it aside. It does not need to be completely cooled to use in the recipe.

Make the blondie batter. Combine eggs with both sugars and vanilla in a large mixing bowl and whisk vigorously until pale and thick. It should double in volume. You can also use a hand mixer for this step. Mix in salt and then mix in the melted white chocolate mixture. Sprinkle in the flour and fold it in gently. Add dark chocolate chunks and chopped roasted hazelnuts and fold them in evenly.

Spread batter evenly into your prepared pan. Sprinkle a few extra chocolate chunks and hazelnuts over top and then bake for 23-26 minutes until the top is shiny, golden and crackly and the center is still soft. Do not over bake these. Transfer pan to a wire rack and let the blondies cool completely before slicing. Enjoy with a glass of cold milk!