Preheat the oven to 350°F (180°C). Spray a 9x9 square dish with non-stick spray and line it with parchment paper so that there is overhang on either side. Set aside.
Add melted butter and brown sugar to a mixing bowl and whisk together.
Add eggs, one at a time, stirring well after each egg is added, and then add the egg yolk and vanilla and stir in well.
Sift all purpose flour into a separate mixing bowl and add cornstarch, baking powder and salt and mix together.
Slowly add the dry ingredients in with the wet and stir in until combined.
Add white chocolate chips and chopped macadamia nuts and fold in.
Transfer to your prepared 9x9 square dish and smooth down.
Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean or with moist crumbs (no wet batter).
Allow to cool completely in the pan.
When cool, melt the white chocolate for the topping. If the chocolate is too thick to drizzle, then add a teaspoon of coconut oil and stir in. Drizzle the top of the blondies with the melted white chocolate and sprinkle the chopped macadamias on top. Allow to set.
Remove the blondies from the pan and cut into squares.