As an afterthought to Cookie Customizations, I would like to add that I follow the baking times like other folks follow speed limits. Baking times are a mere wisp of a suggestion; a rather general guideline to be adjusted wildly a conditions call demand.
I rarely bake cookies for the actual baking time. Preferring a soft cookie with a wee bit of a crust, I generally bake them for a few minutes less, and occasionally I bake them at 25° lower, than called or by the recipe.
Hence the test cookie. Your oven may be hotter or cooler than average. Your baking sheets may result in faster or slower browning of the cookies. But your cookies will be done when they are done, and they will be burnt mere moments later. So bake one single cookie at the start of your batch, and watch it like a hawk. When the test cookie is done to your satisfaction, bake the rest of the cookies at that time and temperature. (The remaining cookies may actually need just a wee bit more baking time due to the extra dough mass in the ove, but the test cookie data gives you an excellent starting point. )