Preheat oven to 350 degrees F. Line a 9x9-inch baking pan with foil or parchment and spray with non-stick cooking spray.
In a medium bowl, whisk together flour, baking powder, and salt.
In a large bowl, whisk together the butter and brown sugar. Add eggs and vanilla, mixing until well combined. Gradually add in dry ingredients and mix until just combined. Stir in white chocolate chips and pecans.
Spread the batter into the prepared pan. Bake in preheated oven until a toothpick inserted in the center comes out clean, about 35 to 40 minutes. If desired, top immediately with a little flakey sea salt. Remove pan to wire rack and cool completely before cutting.
Notes
If you prefer walnuts to pecans, feel free to substitute an equal amount of chopped walnuts for the pecans in this recipe.