Position a rack in the center of the oven and heat the oven to 325 degrees F. Line the bottom and sides of a 9x13-inch baking pan with foil, leaving some overhang on the sides, and spray with cooking spray.

In a medium saucepan over medium heat, whisk the butter until it is melted. Remove the pan from the heat and add the sugar, cocoa powder, and salt. Whisk until well blended, about 1 minute. Add the eggs and vanilla and whisk until smooth. Add the flour and stir with a rubber spatula until blended. Scrape into the prepared pan and spread evenly. Bake until the top is shiny and dry-looking and the brownie springs back very slightly when pressed with a fingertip, about 20 minutes. (The brownie should not be completely baked.) Remove from the oven and put on a rack.

In a large mixing bowl, combine the brown sugar and flour. Whisk until well blended, breaking up any large clumps. Add the corn syrup, melted butter, and vanilla. Whisk until blended, about 1 minute. Add the eggs and whisk just until combined, about 30 seconds. (Don't overmix or the batter will be foamy.) Add the nuts and coconut and stir with a rubber spatula until evenly blended.

Pour the macadamia topping over the warm, partially baked brownie layer. Using a spatula, carefully spread the mixture into an even layer. Return the pan to the oven and bake until the top is golden brown, 37 to 40 minutes. Transfer the pan to a rack to cool completely.

Using the foil as handles, lift the rectangle from the pan and invert onto a work surface. Carefully peel away the foil. Flip right side up. Using a sharp knife, cut into 2x2-inch squares and then cut each square into triangles.

Make Ahead: After the brownie and macadamia layers have been baked and cooled, the entire pan can be wrapped in plastic wrap, then foil, and frozen for up to 1 month.

For the best results, measure your flour by weight instead of volume. (1 cup of all-purpose flour equals 4-1/2 ounces) If you don't have a scale, be sure to use the proper technique when filling your measuring cups.

Dipping the knife in warm water and wiping it dry between cuts will keep the gooey topping from sticking to the knife.