Preheat the oven to 350°F (180°C).
Line a 9 x 9-inch baking pan with foil or parchment paper, leaving enough overhang to help you lift the blondies from the pan when they're done. Spray lightly with cooking spray.
Whisk the flour, baking powder, & salt together in medium bowl; set aside.
Whisk the melted butter & brown sugar together in medium bowl until combined. Whisk in the zest and orange juice.
Add the eggs & vanilla and mix well.
Using rubber spatula or wooden spoon, blend dry ingredients into egg mixture until just combined; do not overmix. Fold in chocolate and nuts.
Spread batter into prepared pan & smooth all the way to the edges & into the corners evenly.
Bake until top is shiny, cracked, and light golden brown - this should take 25-30 minutes but I have had to add up to 10 more minutes of baking time depending on the oven. Just keep an eye on them.
The blondies should be set in the center but still moist as they will continue to set as cooling. Cool on wire rack to room temperature. Remove bars from pan using foil and cut into 16 squares and serve.