Grease and line an 18 X 28cm slice tin with baking paper and set aside.
Crush the biscuits by pressing Turbo for 2-3 seconds. Set aside.
Place the butter, sweetened condensed milk and dark chocolate melts into the Thermomix bowl. Melt on 80 degrees, 3 minutes, Speed 2.
Add the crushed biscuits, coconut, walnuts and cocoa powder and mix on Reverse, Speed 4, 5 seconds. Scrape down the sides of the bowl and repeat for a further 5 seconds.
Spread the mixture evenly into the prepared tin, pressing down firmly.
Melt the dark chocolate and vegetable oil in a clean and dry Thermomix bowl on 60 degrees, 3 minutes, Speed 2. 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.