Crush the Oreos into fine crumbs. You can use a food processor or just pop them in a ziplock and hammer away.
Combine Oreo crumbs with melted butter, press into the bottom of a 9x13-inch dish. Place in fridge to set.
Beat the cream cheese until fluffy. Add sugar and 2 tbsp milk, beat again.
Fold in 1 and 1/4 cups of Cool Whip, then spread this over the Oreo crust.
In a separate bowl, whisk chocolate pudding mix with 3 1/4 cups milk until it thickens, about 2 minutes.
Spread pudding layer over cream cheese layer. Let sit for 5 minutes to firm up slightly.
Top with remaining Cool Whip and sprinkle mini chocolate chips on top.
Chill in fridge for at least 4 hours (overnight is even better). Slice and serve cold!