Grease and line a 20 x 20cm (8 x 8") baking tin, leaving extra baking paper overhanging to help you remove the slice from the tin, and set aside.
Place the biscuits (375g) into a food processor and process into fine crumbs.
In a small saucepan, combine the condensed milk (395g), salted butter (100g), vanilla (½ tsp), and roughly chopped Mars Bars (4) and melt together over low heat until smooth.
Add condensed milk mixture to the crushed biscuits and mix to combine.
Press base mixture into the prepared tin and smooth out the top.
Place the Caramilk chocolate (180g) and vegetable oil (1 tbsp) in a small microwave-safe bowl and melt in the microwave in 30 second bursts (mixing with a metal spoon in between bursts) until just melted, then mix well to combine the oil and chocolate.
Pour the melted Caramilk evenly over the base and allow to set in the fridge.