Preheat the oven to 350°F. Butter the bottom and sides of a 9-by-13-inch baking pan. Put a long piece of parchment paper in the bottom of the pan, letting the parchment extend up the two short sides of the pan and overhang slightly on both ends. Butter the parchment.
In a medium bowl, whisk together the flour, baking powder, and salt; set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream butter and brown sugar until light and fluffy. Add eggs and vanilla, blend until light in color, about 1 to 2 minutes.
Add flour mixture to creamed mixture and blend just until combined.
Stir in 1/4 cup macadamia nuts, 1/2 cup white chocolate, and toasted coconut, spread into prepared pan. Sprinkle with 3/4 cup coconut and remaining nuts.
Bake 34 to 36 minutes, or until a tooth pick inserted in the center comes out clean. Cool blondies completely in pan on wire rack.
Melt remaining ½ cup white chocolate in a metal bowl over a pan of lightly simmering water. Stir occasionally until smooth, and remove from heat. Transfer into a ziploc bag with the corner snipped off and drizzle over blondies. Once chocolate has set, remove blondies from pan and cut into 24 bars.