Cut a piece of parchment paper so that it fits into your an 8×8 square baking pan, leaving some hanging over the edges. Place it in the pan and fold the parchment that is overhanging to form a crease in the shape of the square pan.
Remove the parchment from the pan and place it on a flat surface like a cookie sheet, cutting board, or cooling rack. Spread the chocolate hazelnut spread onto the parchment in the shape of the square pan, so that it will fit into the baking pan in one piece. Freeze for around 20 minutes, then begin making the blondies.
Preheat the oven to 350°F and line with parchment paper. Leave parchment paper hanging over the sides of the pan so that you can easily remove the blondies from the pan later.
Melt the butter in the microwave or in a small pan over the stove. Transfer the melted butter to a medium sized bowl.
Add in both the sugars and use a whisk to whisk the butter and sugars together. Keep whisking until the mixture resembles a paste that pulls away cleanly from the side of the bowl when you mix it. This helps create the crinkle top!
Add the eggs and vanilla and mix again.
Pour the flour and salt into the bowl and mix with the whisk until just combined (do not over mix!).
Pour half of the batter into the lined baking pan, then take the chocolate hazelnut spread out of the freezer, peel it from the parchment (it should come off easily and in one piece*), and place the whole piece on top of the blondie batter.
Pour the remaining blondie batter on top of the cookie butter into the pan and smooth into an even layer.
Bake for 30-45 minutes*. Begin checking the blondies at 30 minutes by inserting a toothpick in the middle. The top of the blondies should be crinkly and the toothpick should not be covered in batter. The blondies are ready when the toothpick has a few wet crumbs.
Let the blondies cool completely before slicing. Enjoy!