Preheat the oven to 350°F. Line a 9x13-inch baking dish with parchment paper and set aside.
In a large bowl, combine butter and both sugars. Use a wooden spoon to stir well.
Add eggs, egg yolk, and vanilla. Mix well and set aside.
In a separate bowl, sift together flour, cornstarch, salt, and baking powder. Slowly add to the butter mixture. Stir until just combined.
Fold in white chocolate chips and walnuts, then spread the batter evenly into the prepared pan.
Bake for 25–30 minutes, or until a toothpick comes out clean or with fudgy crumbs.
Remove from the oven and top with extra white chocolate chips, if desired. Let sit in the pan for 10–15 minutes, then remove from the pan and cool completely before cutting.