Set aside 6 biscuits and grind the rest in a food processor or blender. In a small bowl stir together the cookie crumbs and butter until well blended. Press into the bottom of a 20x30cm pan.
Over a double boiler or in the microwave melt 1 cup of chocolate chips together with the condensed milk and vanilla. Stir frequently until smooth. Spread evenly over the crumb crust. Sprinkle the remaining cup of chocolate chips over the top. Break remaining biscuits into pieces by hand and sprinkle over the top.
Bake for 20 to 25 minutes in the preheated oven. Chill completely before cutting into bars or it will make a big mess.