Makes 24 pieces
In bowl of electric mixer, whip 1 cup whipping cream until firm peaks form. Add pudding mix, and with mixer on low speed, slowly add chocolate milk. Beat 3 minutes until mixture begins to thicken. Let stand 3 minutes.
Meanwhile, line 13x9x2-inch pan with foil. Place 3 rows of 5 cookies each in bottom of pan. Lay 2 more rows of 5 cookies down the middle of the 3 rows for full coverage. Spread 2 cups of filling on top cookie layer. Cover with another layer cookies, followed by 2 cups filling. Add another layer cookies and top with remaining filling. Add final layer cookies. Cover and refrigerate overnight.
When ready to serve, whip remaining cream with chocolate milk powder. Cut cake into serving pieces; top each with whipped cream and chocolate sauce.