1. Line baking tray with baking paper.
2. Place 120g butter, mars bars and golden syrup in a saucepan over medium heat. Stir constantly.
3. When butter is melted and only small unmelted bits of nougat remain, increase heat and continue to stir constantly.
4. Stir until mixture is thick and smooth.
5. Place rice bubbles into a large bowl and pour mixture over. Stir until rice bubbles are completely covered.
6. Press into pan. Place in fridge.
7. Combine extra chocolate (225g) and 45g butter into a saucepan on low heat. Stir until melted.
8. Pour over rice bubbles and refrigerate until set.