Preheat oven to 160°C fan-forced (180°C conventional). Lightly grease and line a 20 x 30cm lamington pan with baking paper.
In a medium saucepan, melt butter on medium until foaming and golden. Remove from heat, add chocolate, stir until smooth. Set aside for 10 minutes.
In a small bowl of an electric mixer, beat sugar and eggs until thick and pale. Gradually add butter mixture, beating to combine. Fold flour through.
Add ¾ each of the extra chocolate, macadamias and rhubarb, fold through. Pour mixture into pan and smooth top. Top with remaining extra chocolate, macadamias and rhubarb.
Bake for 35-40 minutes until golden. Cool in pan for 1 hour. Cut into squares. Store in an airtight container.
