Line 13 x 9-inch pan with foil. Spray foil with cooking spray. Place 30 of the crackers in bottom of pan.
In 2-quart sauce pan, mix condensed milk, brown sugar, butter and milk. Cook over medium heat until butter is melted, stirring frequently. Increase heat to medium-high' heat to boil. Boil 5 minutes, stirring constantly.
Pour one-third of the caramel mixture over crackers in pan. Arrange 30 of the crackers over caramel. Top one-third of the remaining caramel mixture. Repeat with remaining crackers and caramel.
In small microwaveable bowl, microwave chocolate chips uncovered on high for 1 to 2 minutes, stirring every 30 seconds, until melted and smooth. Pour over caramel; spread evenly. Refrigerate about 1 hour or until set. Cut into 6 rows by 8 rows. Store covered up to 4 days.