Gently melt the butter, chocolate and golden syrup in a heavy-based saucepan. Scoop out about 125ml / ½ cup of this melted mixture and put to one side.Put the biscuits into a freezer bag and then bash them with a rolling pin. You are aiming for both crumbs and pieces of biscuits.Fold the biscuit pieces and crumbs into the melted chocolate mixture in the saucepan, and then add the marshmallows.Tip into a foil tray (24cm / 9 inch square); flatten as best you can with a spatula. Pour the reserved 125ml / ½ cup of melted chocolate mixture over the marshmallow mixture and smooth the top.Refrigerate for about 2 hours or overnight.Cut into 24 fingers and dust with icing sugar by pushing it gently through a tea strainer or small sieve.