1. Preheat oven to 350 degrees. Prepare a bar pan (about 10x15) with nonstick cooking spray or parchment paper.
2. In a large bowl, whisk together the flour, baking powder, and salt.
3. In a mixing bowl, cream the butter and sugars until fluffy, about 3 minutes. Add the eggs and vanilla, mixing until combined.
4. Add the dry ingredients and mix well. Stir in the white chocolate chips.
5. Transfer batter to bar pan and spread to smooth. Top with additional white chocolate chips if desired.
6. Bake for 20 to 25 minutes or until a toothpick inserted in the center returns clean. For best results, cool completely before slicing.