Add the butter and the Carnational caramel to a medium non-stick saucepan. Place over a medium heat and allow the butter to melt, while occasionally stirring with a wooden spoon. Once melted, bring the caramel to the boil then turn the heat down to a gentle simmer (small bubbles only) and cook for a further 8 minutes.

Tip the rice crispies into the pan with the caramel and stir until all the rice crispies are well coated with caramel.

Tin a 20cm square tin or dish with non-stick baking paper (you may also wish to lightly grease the tin to stop the paper sliding around). Tip the caramel rice crispies into the tin and use a fork to press them into an even layer across the base. Cover with cling film and place in the fridge to chill for 30 minutes.

Once chilled, chop the milk chocolate into small pieces and place in a microwave-safe bowl. Microwave for 30 seconds, leave to sit for a minute then stir. Repeat this once or twice, until the chocolate is melted. Pour the melted milk chocolate over the chilled rice crispie base.

Chop the white chocolate into small pieces and place in a microwave-safe bowl. Melt per the process in step 4. Then drizzle it over the milk chocolate and then drag and swirl a skewer or fork through the two chocolates to create a marbled effect.

Cover with cling film and place in the fridge to chill for at least 30 minutes or until the chocolate has just set but is not completely chilled - it's much easier to cut them before the chocolate goes rock solid. Now, remove from the tin and cut into 12 or 16 pieces (depending how big you want the millionaires to be). Then pop them back in the fridge until you want to eat them. They're keep for up to 5 days.