Preheat the oven to 350°F. Spritz a 13 x 9 inch non stick pan with baking spray. Set aside.
To toast the nuts: Spread the walnuts in a single layer on a baking sheet. Toast for 6-8 minutes at 350°F then cool completely before adding to the batter.
Sift together the all purpose flour, baking powder and salt. Set aside.
In a medium sauce pan, melt together the butter and brown sugar over medium heat. Cook for 2-3 minutes until the sugar has melted, stirring constantly.
Remove from the heat and pour into a large mixing bowl. Cool just until it's warm to the touch.
Use a hand mixer to beat in the vanilla and the eggs one at a time. Gradually add the dry ingredients and continue to beat until moistened. The batter will be stiff.
Mix-in half of the butterscotch chips and all of the toasted walnuts by hand.
Spread the batter into the pan and sprinkle the top with the remaining butterscotch chips. Press gently into the batter.
Bake for 30 minutes or until a toothpick inserted into the center comes back with moist crumbs and the top is lightly golden.
Cool to room temperature before attempting to cut into squares and remove from the pan. Store in an airtight container at room temperature.