Crush the biscuits until they resemble breadcrumbs.
Place biscuits in a medium-large bowl, add the coconut and set aside.
In a heat-proof bowl, melt the butter in the microwave and then add the condensed milk and vanilla extract and stir.
Add the butter mixture to the biscuit crumbs and then mix until it resembles wet sand.
Place the mixture into the prepared slice tin and push down firmly with the back of a spoon. Place into the fridge.
Meanwhile, melt the coconut oil and chocolate together in the microwave in a microwave-safe bowl on 50% power, 2-3 minutes (stirring every 30 seconds) or until melted.
Once melted, pour over the slice and sprinkle with extra coconut (if desired).
Place back into the fridge to set.