Preheat the oven to 350 degrees Fahrenheit and line a 9 x13 x 2-inch baking pan with parchment paper, leaving a 1-inch overhang on either side. Lightly spray the parchment paper with nonstick cooking spray. Set aside.
Add 1 cup melted unsalted butter , 1 cup packed light brown sugar, and ¾ cup granulated sugar to a large bowl and mix until combined.
Add 2 large eggs and 1 egg yolk one at a time, mixing well after each addition. Add 1 ½ teaspoon vanilla extract and mix until combined.
Add 2 ½ cups all-purpose flour, 2 teaspoons cornstarch, ½ teaspoon baking powder, and 1 teaspoon salt to a medium bowl and whisk until combined.
Gradually stir the flour mixture into the sugar mixture until fully combined.
Spread the batter evenly into the prepared pan, then drop ½ cup Nutella in small spoonfuls all over the top of it. Use a butterknife to swirl the Nutella through the batter.
Bake for about 25 minutes, just until the edges begin to turn golden brown and a toothpick inserted in the center comes out clean.
Let the blondies cool completely before slicing and serving. to cool before slicing and serving.