Heat oven to 350 degrees and set rack to middle position. Line a 9X13 baking pan with parchment or foil (leave enough to overhang so it's easy to lift the blondies out of the pan after they've cooled). Spray the parchment/foil with vegetable spray. Set aside until ready to use.
Spread nuts onto a baking sheet and bake approx. 10 minutes, until nicely brown and fragrant. When done, transfer to a cutting board to cool, then coarsely chop.
While nuts roast, place butter in a small bowl and microwave for 30 seconds at a time until fully melted. Remove and allow to cool.
Whisk flour, baking powder and salt in a bowl.
In a separate, large mixing bowl, whisk melted/cooled butter and brown sugar until smooth, then add eggs and vanilla and whisk again. Gently fold in dry ingredients with a rubber spatula. When just about combined, add white chocolate and nuts and mix again. Don't over mix!
Spread batter into prepared baking pan. Bake 22-25 minutes, until a toothpick comes out clean in the center and the top is shiny and slightly golden brown. Remove from oven and allow to cool to room temperature. Remove from pan by lifting parchment/foil. Transfer to a cutting board to cut into 2" pieces (or smaller or larger, to your liking). Store leftovers in an airtight container.