Serves: 24 squares1) Melt the butter, chocolate and syrup in a heavy based saucepan. Scoop out about 110ml of this melted mixture and put aside.
2) Put the biscuits into a freezer bag and then bash them with a rolling pin. You are looking for both crumbs and pieces of cookies.
3) Fold the biscuit pieces and crumbs into the melted chocolate mixture, and then add the marshmallows.
4) Pour into a 23cm square foil tray and flatten with a spatula. Pour over the reserved 110ml of melted chocolate mixture and smooth the top.
5) Refrigerate for about 2 hours or overnight. Cut into squares to serve.
6) Cut into 24 fingers and dust with powdered sugar by pushing it gently through a tea strainer or small sieve.
Recipe from Nigella Express by Nigella Lawson (published by Chatto & Windus) Visit Nigella's site.