1. Preheat the oven to 160°C.
2. Grease and line the base of a 20cm rectangle pan.
3. Place the butter and chocolate in a microwave safe bowl and melt in the microwave. To ensure you do not burn the chocolate, I just put it on for 20 seconds, stir it and then continue this until it has melted.
4. When butter and chocolate have melted, stir through the cocoa powder. Allow to cool slightly.
5. Beat the sugar, vanilla and pinch of salt into the mixture.
6. Whisk in the eggs one at a time, beating well after each addition, then add egg yolk and flour, beating until smooth.
7. Fold in the macadamias and pour mixture into pan, smoothing the top.
8. Bake for 40 minutes or until a skewer comes out with a few moist crumbs attached (not sticky batter).
9. Allow to cool, then turn onto a wire rack and peel off lining paper.
10. Cut into squares and dust with cocoa powder or icing sugar before serving.