Heyo! Ok so <MY Opinion> :The cream, because the small chocolate outsides are meant to add a balance between the cream, because if cream was all the cookie contained, it wouldn't taste as delicious. Obviously you have noticed that theres usually more cream than the chocolate outsides because the cream is the primary source of the tatse, Milk just adds even more to the taste of Oreos! :) Hope this helped ! have a good day!