Preheat oven to 350F. Line an 8-by-8-inch baking pan with aluminum foil, spray with cooking spray; set aside.
To a large, microwave-safe bowl add the butter and heat to melt, about 1 minute on high power.
Wait momentarily before adding the eggs so you don’t scramble them. Add the eggs, brown sugar, vanilla, and whisk until smooth.
Add the flour, salt, and stir until just combined, don’t overmix.
Turn half the batter out into prepared pan; set remaining batter aside.
To a medium, microwave-safe bowl add the Nutella and heat to soften, about 10 to 15 seconds on high power. Take care not to melt it, just soften it. Heating makes it easier to spread in step 7. You can also microwave the jar of Nutella itself if you’re extra careful to peel away the foil seal.
Turn Nutella out into the pan and spread into an even, smooth layer.
Turn remaining batter out over the Nutella. I find it easiest to add heaping rounded tablespoon-sized blobs and use a spatula to smooth them together.
Evenly top with chocolate chips.
Bake for about 35 minutes, or until top is set in center and done; don’t overbake as they taste best when they’re slightly underbaked. A toothpick test is unreliable because you will hit melted Nutella. Blondies firm up as they cool.
Place pan on a wire rack to cool for at least 90 minutes before slicing and serving.
Notes
Blondies will keep airtight at room temperature for up to 1 week, or in the freezer for up to 6 months.