Grease a slice tin (around 27 x 17cm) with butter and then line with baking paper, leaving extra on each side (to pull the slice from the tin once cooked).
Measure rice bubbles into a large mixing bowl.
Break mars bars into pieces with your hands and put into a saucepan. Add butter and golden syrup.
Place saucepan on medium heat while stirring to combine.
Keep stirring until the mixture reaches a smooth, runny consistency. Use a whisk to stir the mixture towards the end of the process to help get that completely smooth consistency. The mixture should resemble melted chocolate, with no lumps.
Add mixture to rice bubbles and stir quickly to combine.
Spread out mixture in the slice tin, spreading evenly to the edges and then pressing into the tin with the back of a metal spoon. Place in fridge for 1/2 hour.
Melt chocolate in the microwave using 15 second bursts. (check and stir after each burst).
Pour melted chocolate over the slice and angle tin from edge to edge to help chocolate reach each side and corner of the slice tin.
Place in fridge for 15-30 minutes. Ideally, the chocolate shouldn't be completely set when cutting (it will crack as you're cutting if it's too hard).
Once set, remove slice from tin by lifting edges of baking paper, cut into squares and return to the fridge. (And if you've left the slice in the fridge for longer, just let the slice rest at room temperature for half an hour before cutting into squares.)