20 mins prep. time
What you need:
Use a little of the butter to grease the inside of a round tin (I used a 20cm (8 inch) tin). Put the rest of the butter in a saucepan with the condensed milk and chocolate. Heat gently, stirring until melted, then remove from the heat.
Place the biscuits in a plastic bag and crush into crunchy pieces. Chop the pistachio nuts. I suggest removing the shells of the pistachio nuts before doing anything else, it takes an annoyingly long amount of time.
Stir the biscuits into the chocolate mixture then spoon the mixture into the tin making sure it is level. Reserve some nuts for later and sprinkle the rest over the chocolate biscuit layer. Cover with the remaining chocolate biscuit mixture and level the surface. Sprinkle the rest of the nuts on top.
Chill for 3 hours. Loosen the edges and cut into bite sizes pieces.