Line a 24cm x 30cm slice tin with baking paper and set aside.
In a small saucepan, heat the dark chocolate, butter and condensed milk over a low heat for 5 minutes or until the mixture has melted. Set aside for a few minutes to cool.
Meanwhile, place the biscuits into a plastic bag. Use the end of a rolling pin to carefully crush the biscuits. Place the broken biscuits in a large bowl and add the coconut, walnuts and cocoa.
Pour the melted chocolate mixture into the dry ingredients, and mix well until everything has combined.
Press firmly into the prepared tin with the back of a wooden spoon, and refrigerate for 30 minutes or until firm.
To make the chocolate topping, place the butter and chocolate in a heatproof bowl over a saucepan of simmering water. Stir regularly for 5-8 minutes or until the chocolate and butter have melted and combined.
Pour the chocolate mixture over the slice and refrigerate for 1 hour or until completely set.
Cut the slice into squares and serve. Store in an airtight container in the fridge.