Grease and line an 18cm x 28cm slice tin with baking paper and set aside. Microwave butter, sweetened condensed milk and dark chocolate melts until melted. Stir well. Add the crushed biscuits, coconut, walnuts and cocoa powder and mix together well.
Melt the dark chocolate melts and vegetable oil until just melted.
Pour the melted chocolate over the top of the slice and refrigerate for a minimum of 3 hours. Cut into slices and store in an airtight container in the fridge for up to 5 days.