Line a 28 x 18cm tin with baking paper. Make sure you leave plenty of paper hanging over the sides to help you remove the Mars Bar Slice once it has set.
Place the butter, golden syrup and 4 of the Mars Bars (which have been chopped into quarters) into your Thermomix bowl and cook for 3 minutes, 60 degrees, speed 3. Scrape down the sides and cook for a further 1 minute if the ingredients haven't quite melted.
Add the rice cereal and mix for 10 seconds on speed 4 REVERSE. Scrape down the sides of the bowl and mix for a further 5 seconds on speed 4 REVERSE or until the rice cereal has been covered with the melted Mars Bar mixture.
Pour the mixture into your prepared slice tin and push down with the back of a metal spoon to flatten and smooth the surface.
In a clean Thermomix bowl, add the milk chocolate (which has been broken into pieces) and cook for 2 minutes at 50 degrees on speed 3. Scrape down the sides and cook for a further 1 - 2 minutes or until the chocolate has completely melted.
Pour the melted chocolate over the top of your Mars Bar Slice and decorate with pieces of the extra Mars Bar and place into the fridge for 4 hours to set.
Cut the set Thermomix Mars Bar Slice into pieces and enjoy!