Preheat oven to 350°. Line a 9-inch square baking pan with foil, letting excess extend over sides of pan; spray foil with cooking spray.
In a medium bowl, whisk together melted butter and sugars until smooth. Whisk in eggs and vanilla. In another medium bowl, whisk together flour, baking powder, and salt. Gradually whisk flour mixture into egg mixture just until combined. Spread batter in prepared pan.
Place Nutella in a small microwave-safe bowl; cover and heat on high for 10 seconds. Stir until smooth. Drizzle Nutella in wide ribbons onto batter; swirl together with a knife, avoiding bottom of pan.
Bake until golden brown and a wooden pick inserted in center comes out clean, 20 to 25 minutes. Let cool in pan for 10 minutes. Using excess foil as handles, remove from pan, and let cool completely on a wire rack. Cut into 3-inch squares; store in an airtight container for up to 3 days.
Notes
Creamy peanut butter can be substituted for Nutella.
Recipe by Paula Deen Magazine at https://pauladeenmagazine.com/chocolate-hazelnut-blondies-recipe/