Whisk butter with caster sugar and icing sugar until pale white. Add vanilla essence, egg and zest and combine.
Add chocolate and macadamia nuts. Sift in the flour and baking powder and combine with a spatula, by adding one-third or half of the flour at a time to make it easier. Don’t over whisk the mixture, just to combine all the ingredients. (It will be slightly sticky.) Transfer to freezer for 25 to 30 minutes, until it turns hard enough to knead, but not yet frozen.
Preheat oven to 160C (315F).
Remove the dough from freezer and transfer onto a baking paper. Sprinkle some flour and shape into a log about 5cm wide and 15cm long. Transfer onto a baking tray. Bake for 30 to 35 minutes, or until firm to touch and just cooked through. Cool on tray.
Reduce the oven to 150C (300F). Cut the log diagonally into 1cm thick slices with a sharp knife. Place on the lined baking tray in a single layer. Bake for 20 minutes, or until the biscotti are crisp. Transfer the biscotti to a wire rack to cool completely.
The thinner the biscotti you cut, the shorter cooking time it takes, the crispiest biscotti you’d get. Be careful the thinner biscotti will be easier to be broken into pieces.
If you’re already satisfied with the colour on the biscotti, but they are not yet crispy enough or not up to your preference. Simply turn off the oven and leave the biscotti inside the oven until they dry up completely and turn crispy.