Preheat the oven to 350 degrees F. Cut a piece of foil 9 by 20-inches. Butter the corners and sides of a 9-by-13-inch baking pan. Line the pan with the foil so that the excess hangs over at each of the short sides of the pan.
Whisk the flour with the baking powder and salt in a medium bowl.
Whisk the eggs, sugars and vanilla until smooth in a large bowl. Using a rubber spatula, stir in the melted butter. Stir the flour mixture into the butter mixture until just combined. Stir in the white chocolate chips and nuts. Spread the batter in the prepared pan using a large spoon. Sprinkle the coconut over the top and press it in lightly. Bake until the top of the blondie is golden and set, the coconut is very brown and a toothpick inserted in the center comes out clean, 25 to 30 minutes. Cool completely on a rack, about 1 hour.
When the blondie has cooled completely, place a lightweight cutting board or a baking rack over the top of the pan. Flip and set on the counter. Gently pull the pan from the foil and the foil from the pan to remove. Gently peel the foil off the blondies. Place another lightweight cutting board on top, and flip the blondie right-side-up. Slice into fifteen 2 1/2-by-3-inch rectangles, then slice each rectangle on the diagonal to form 30 triangle-shaped bars.
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