Preheat oven to 350 degrees F. Line a 13- by 9-inch metal baking pan with foil, with foil edges hanging over the side. Spray the foil with cooking spray.
In a medium bowl, whisk together flour, baking powder and salt.
In a large bowl, whisk together melted butter, brown sugar, eggs and vanilla extract. Stir flour mixture into sugar mixture until just combined. Add white chocolate chips and stir to combine.
In a medium bowl, stir together cream cheese, Nutella and egg yolk until completely combined.
Transfer blondie mixture to prepared pan and spread evenly. Drop the Nutella mixture by tablespoonfuls over the blondie mixture. Using a butter knife, swirl in the Nutella mixture.
Bake, rotating halfway through, until a toothpick inserted into the middle comes out with a few crumbs, 25 to 28 minutes. When testing, be sure to insert the toothpick into a section with blondie, rather than Nutella.
Cool the blondies, in the pan, on a wire rack until completely cool, about 2 hours. Holding the overhanging foil, lift out the blondies. Cut into squares and serve.