Begin by heating the oven to 350°.
In a large bowl mix together the butter, brown sugar, egg, and vanilla. Then stir in the baking powder, baking soda, salt, and flour until all of the ingredients are well-blended.
Fold in the white chocolate chips and chopped nuts.
The batter should be pretty thick. Pour it into a greased 9x9 inch pan, and cook for 20 to 25 minutes.
While the blondies are cooking, you can prepare the maple butter sauce.