Grease the tin with oil and line with parchment paper, leaving 5cm excess hanging over the edges to help with lifting out later.
Snap 300g of the chocolate into a large saucepan and add the condensed milk.
Warm together over a low heat, stirring regularly until the chocolate has melted.
Meanwhile, blend the biscuits in a food processor, or seal in a food bag and bash with a rolling pin, to fine crumbs.
Stir into the chocolate mix until well blended.
Working fast before the mixture sets, spread into the tin with the back of a spoon. Set aside.
Toast the chopped macadamia nuts in a dry frying pan over a medium heat, tossing regularly, until golden.
Tip into a small bowl and leave to cool.
Break the remaining 100g of chocolate into a small heatproof bowl.
Either melt in the microwave in 30 second blasts, stirring between each go, or over a pan of simmering water, shallow enough that the water doesn’t touch the bowl.
Once melted, remove from the heat and stir until smooth.
Pour over the biscuit base and spread evenly with the back of a spoon.
Toss the cranberries in with the nuts and then scatter over the chocolate.
Cover with cling film and chill in the fridge for at least an hour or overnight, until firm.
Remove from the tin and peel off the paper.
Cut into eight rectangular bars. These can be halved again for 16 smaller squares if you prefer.