Preheat the oven to 350 degrees F. Line a 9×13-inch pan with foil (optional), lightly grease with cooking spray (not optional.)
In a medium microwave-safe bowl, melt the butter. Add the brown sugar and mix well. Next, add the eggs and vanilla and mix. Add the flour, baking powder, salt and baking soda and blend until combined. Mix in white chocolate chips.
Spread batter in the prepared pan. Bake for 25-30 minutes until a toothpick inserted in the center comes out clean or with a few moist crumbs.