The secret lies in the length of time you bake them. Never bake them as long as the recipe calls for! Bake them until JUST beginning to get golden on the top....JUST until the inside is not runny. Sometimes it's even half of what the recipe calls for depending on your oven. All ovens are different regarding how long baking is required.
That's my "expert" advice for which I charge 1 large warm & gooey cookie.