Preheat oven to 350°F. Line a 9x13 pan with a parchment sling or spray well with baking spray.
In a small bowl, whisk the flour, baking powder, baking soda, and salt together. Set aside.
2 cups all-purpose flour, 1 teaspoon baking powder, ½ teaspoon baking soda, ¼ teaspoon kosher salt
In a large bowl, use a handmixer on low speed to combine the melted butter and brown sugar until fully combined, about 30 seconds.
10 tablespoon butter, 2 cups brown sugar
Add eggs and vanilla and mix on medium-low speed for 1 minute. Scrape bowl.
2 eggs, 2 tablespoon vanilla
Add half the flour mixture and mix on low until just combined. Scraped the bowl and repeat with the remaining flour mixture.
Add white chocolate chips and walnuts and mix on low until just combined. Don't overmix.
1 cup white chocolate chips, 1 cup walnuts
Spread batter evenly into prepared pan (the batter will be thick). Bake for 35-40 minutes, until the top is golden brown and a toothpick inserted in the center comes out clean or with moist, fudgy crumbs. Allow to cool fully in the pan.