Preheat the oven to 350°F. Spray a 9×13 dish with non-stick spray.
Mix together the flour, baking powder, baking soda and salt. Sift the ingredients into a bowl. Add chopped nuts and mix well; set aside.
In a separate bowl, add the brown sugar to melted butter and mix well. Then add egg and vanilla extract and mix again.
Gradually add the dry ingredients to the wet ingredients, mixing between additions and until completely combined.
Stir in the white chocolate chips and spread out dough in a 9×13 inch pan. The batter is thick – we usually spread it with a rubber spatula. Bake for 20 to 25 minutes. Use a toothpick or fork to test if it is cooked in the center.
