Grease and line a 28cm x 18cm (11" x 7") rectangular slice tin with baking paper (leaving some overhang) and set aside.
In a large bowl, place the sweetened condensed milk, salted butter and Caramilk chocolate.
395 g sweetened condensed milk, 80 g salted butter, 180 g Caramilk chocolate
Microwave in 30-second bursts (stir between each) to melt the chocolate and combine.
Add vanilla, crushed biscuits and coconut and mix well to combine.
1 teaspoon vanilla, 250 g plain sweet biscuits (I used Arnott's brand Marie biscuits), 85 g desiccated coconut
Tip the base mixture into the tin, pressing into an even layer and smoothing the top (use the back of a spoon or a silicone spatula for this).
To make the chocolate topping layer, place Caramilk chocolate and vegetable oil in a small bowl.
180 g Caramilk chocolate, 1 tablespoon vegetable oil
Microwave in 30-second bursts (stir between each) to melt and combine.
Pour the chocolate mixture over the base and place in the fridge to set.