Why are some of the cookies perfectly done while others are doughy and some are too brown?

To ensure even baking, use a cookie sheet that fits in the oven with at least 1 inch to spare all around. Bake only one cookie sheet at a time, and be sure the rack is in the middle of the oven. Be careful to place the cookies on the cookie sheet exactly as specified in the recipe. The spacing has been worked out to allow for spreading of the dough and even airflow around the cookies.