Cut a piece of parchment paper to fit an 8×8 square baking pan, leaving some hanging over the edges. This will make it easier to lift the blondies out later.
Place the parchment paper on a flat surface (like a cutting board or cookie sheet) and spread the chocolate hazelnut spread onto it in the shape of the pan. Freeze for about 20 minutes while you make the blondie batter.
Preheat the oven to 350°F (175°C) and line the baking pan with a new piece of parchment paper, leaving some overhanging for easy removal.
Melt the butter and transfer to a medium-sized bowl. Whisk in both the light brown sugar and granulated sugar until the mixture becomes thick and paste-like.
Add the eggs and vanilla extract, whisking until well combined.
Gently fold in the flour and salt, mixing just until combined—be careful not to over-mix!
Pour half of the blondie batter into the prepared baking pan.
Remove the frozen chocolate hazelnut spread from the freezer, peel it off the parchment, and place it on top of the batter.
Pour the remaining blondie batter on top, smoothing it out into an even layer.
Bake for 30-45 minutes, checking the blondies at the 30-minute mark. Insert a toothpick into the center—if it comes out with a few moist crumbs but not batter, the blondies are ready.
Allow the blondies to cool completely before slicing. Enjoy your gooey Nutella Blondies!