In a large food processor, crush Oreo cookies into fine crumbs. Make sure there are no large chunks left.
In a medium bowl, mix melted butter into cookie crumbs using a fork. When the butter is thoroughly mixed in, transfer it to a 9 x 13 inch baking dish. Using a spatula, press the crumbs down into the bottom of the baking dish as evenly as possible. Put baking dish in refrigerator while you work on the next step.
In a medium bowl, mix the cream cheese with an electric beater until fluffy. Add the 2 tablespoons of cold milk and sugar and mix well. With large spoon or spatula, mix in 1¼ cups of the Cool Whip until combined. Spread cream cheese mixture over cookie crust. Return baking dish to refrigerator and let chill for 10 minutes
In a large bowl, mix together chocolate instant pudding and 3¼ cups cold milk with electric beater or whisk until the pudding starts to thicken. Spread the pudding over the cream cheese layer. Return baking dish to refrigerator and let chill for 10 more minutes.
Spread remaining Cool Whip over the pudding layer and sprinkle mini chocolate chips over the top. (Feel free to use more than ½ cup of the chocolate chips if you want more!)
Chill in fridge for 4 hours or freezer for 1 hour