Whisk together melted butter and brown sugar in a large bowl. Add eggs, vanilla, and maple flavoring and mix well.
Combine flour, baking powder, and salt. Stir into creamed mixture. Add nuts and white chocolate chips.
Spread into a well greased and parchment lined 9×13" pan.
Bake at 350° for about 22-25 minutes. Top will be shiny and golden brown. Don't over-bake!
For sauce: Combine all ingredients in a medium saucepan. Heat over medium heat, stirring frequently. Boil for 2-4 minutes. The longer it cooks, the thicker it gets. Keep in mind that it will also thicken as it cools.
Notes
Nutritional information does not include vanilla ice cream.