Preheat oven to 350 degrees. Line an 8x8 pan with aluminum foil and spray with cooking spray.
In a bowl add flour, baking powder, baking soda, salt, and walnuts. Whisk to combine. Set aside.
In a large bowl, mix the melted butter and brown sugar with a mixer. Add the egg and vanilla and mix well. Add the flour mixture a little at a time until combined. Stir in white chocolate chips. Spread into the 8x8-inch pan and bake for 35-40 minutes or until the center passes the toothpick test. (I took mine out at 35 minutes. If looked done on top but was gooey on the inside, which I like. Every oven varies so test it with a toothpick!)
Serve warm with ice cream and Maple Butter Sauce.
For the Maple Butter Sauce:
In a small bowl add the butter, powdered sugar, cream cheese, and maple syrup. Blend with a mixer until smooth.
When ready to serve, heat the maple butter sauce for about 30 seconds in the microwave and pour over warmed blondie and ice cream.