Line 13x9-inch pan with parchment paper. Then, spray with non-stick cooking spray.
Place about 30 of the crackers in the bottom of the pan.
In a large saucepan over medium heat, add condensed milk, brown sugar, butter, and milk.
Heat until butter is melted, stirring frequently. Increase heat to medium-high; heat to boiling. Boil 5 minutes, stirring constantly.
Pour one-third of the toffee mixture over crackers in pan. Arrange 30 of the crackers over caramel. Top with one-third of the remaining toffee mixture. Repeat with remaining crackers and toffee.
In a microwavable bowl, microwave chocolate chips for 1-2 minutes, stirring every 30 seconds, until melted and smooth.
Pour the melted chocolate over the top of the bars and then use an offset spatula to spread evenly. Refrigerate about 1 hour, or until set.
Store, covered, for up to four days.