Spray a 23cm square cake pan with cooking spray and line with baking paper.
Pre heat oven to 180° celsius.
In the bowl of an electric mixer, cream the butter and sugars until light and fluffy. It takes a few minutes with the mixer on medium high. This could also be done with a hand mixer.
Add the eggs and vanilla and beat at medium speed until combined.
In a separate medium bowl, sift together the flour, baking powder and salt.
Add the dry ingredients to the mix and beat on low until just combined.
Stir through the macadamias and white chocolate chips.
Spread batter into the pan and even out.
Bake for 30 minutes, until golden brown and just cooked.
Allow to cool completely in the pan, then turn out to drizzle with chocolate.