Set aside 6 cookies and grind the rest in a food processor or liquidiser. In a small bowl, stir together the cookie crumbs and butter until well blended. Press into the bottom of a 28x32cm or similar sized tin.
Over a double boiler or in the microwave, melt half of the chocolate chips together with the condensed milk and vanilla. Stir frequently until smooth. Spread evenly over the crumb base. Sprinkle remaining chocolate chips over the top. Break remaining cookies 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.