Preheat oven to 350 F. Spray a 9x9 metal baking pan with cooking spray and line with parchment paper. I cut out a square to line just the bottom of the pan.
Melt the butter in the microwave in 30-second bursts until melted. I find this usually takes about 45 seconds, but be sure to watch the butter so it doesn't explode all over your microwave.
Pour the melted butter (1/2 cup) and brown sugar (1 cup) into a medium-sized mixing bowl and whisk to incorporate.
To the same bowl, add the two eggs and whisk until fully incorporated. Using a spatula, mix in the flour (1 cup), baking soda (1/4 teaspoon), vanilla extract (2 teaspoons), cinnamon and cloves (1/2 teaspoon each), and salt (1/4 teaspoon), if using.
Fold in 1 cup of chocolate chips and 1/2 cup of chopped hazelnuts. Pour into prepared pan and bake for 30 minutes. The blondies should look slightly underdone in the middle when they are done, and will jiggle a bit when you shake the pan. This will give you the delicious, soft center. You can bake them a bit longer if you want more of a firm blondie, but anything over 35 minutes and you will have more of a cookie bar than a blondie.
Remove the blondies from the oven when they are done. The blondies will continue to cook for a bit due to the residual heat from the pan. Let them rest for 10 minutes before turning them out onto a baking rack to continue to cool. I like to flip them over onto a piece of parchment paper first, then put my hand underneath them to help flip them back over.
Slice the blondies into 9 pieces once fully cooled. Enjoy plain or with ice cream and a drizzle of chocolate sauce.