Grease and line an 18cm x 28cm slice tin with baking paper and set aside.
Place the butter, sweetened condensed milk and dark chocolate melts into a microwave-safe bowl. Heat on 50% power, 3-4 minutes, (stirring every 30 seconds), or until melted. Stir well.
Add the crushed biscuits, coconut, walnuts and cocoa powder and mix together well.
Spread the mixture evenly into the prepared tin, pressing down firmly.
Melt the dark chocolate melts and vegetable oil (or coconut oil) in a microwave-safe bowl on 50% power, 3-4 minutes (stirring every 30 seconds), or until just melted. Alternatively, you can use a stove-top for this step.
Pour the melted chocolate over the top of the slice and refrigerate for a mimimum of 3 hours.
Cut into slices and store in an airtight container in the fridge for up to 5 days.