This is an article that I have come back to reference many times through my baking life. In it, the author explores many different variations on the classic chocolate chip cookie recipe and documents the effects that the changes produce on the finished product. Not only are the results documented, but he also explains the science of why that change produces the effect. For example, when discussing how much butter to include:

When flour is mixed with water (such as the water found in eggs), it develops gluten, a tough, stretchy network of interconnected proteins that set up as they bake. Gluten can’t form in fat, thus butter will inhibit its overall formation, leading to more tender results. The higher the proportion of butter to other ingredients, the more tender your cookie will be (and consequently, the more it will spread as it bakes).

As an aside, I cannot recommend The Food Lab by J. Kenji López-Alt enough. It is a phenomenal resource that explains the why behind so many recipes and cooking techniques. Unfortunately, baking is not one of the chapters in the book (so no cookie recipes in it), but it is written in a similar fashion to this article.