Line a 23cm x 33cm rectangular tin (9 x 13 inches)
Crush both packets of biscuits into a fine crumb and mix together in a large bowl.
Melt the butter and condensed milk over a low heat until combined. Keep stirring to avoid the mixture burning.
Once melted, sift in the cocoa and combine.
Pour the liquid over the biscuits and combine until all the biscuit crumbs are coated.
Press into your prepared tin, using the back of a spoon to smooth the mixture out to create an even surface. Refrigerate until set.
Once set, place the chocolate into a heat proof bowl. Place the bowl over a pot of simmering water and allow the chocolate to melt.
Pour the chocolate over the biscuit base and spread evenly. Place back into the refrigerator to set.
Once set, use a hot knife to cut the slice into 24 even pieces.
Store in an airtight container or tin in a cool place.