Preheat oven to 350°F.
In a small bowl, add sifted flour, baking powder, baking soda and salt. Sift again.
Add chopped nuts. Mix well. Set aside.
Melt butter and place into a medium bowl.
Add brown sugar. Mix well.
Add egg and vanilla extract. Blend well.
Add reserved flour mixture, a little at a time, mixing well between each addition.
Stir in white chocolate chips.
Spread evenly in a 9-inch square pan.
Bake, in preheated oven, for 20 to 25 minutes or until a wooden pick inserted in center comes out clean or with slightly fudgy-looking crumbs.
Serve with ice cream and Maple Butter Sauce.