This can be due to too much leavening agent in your cookie. Reducing your baking power/baking soda by half can help with this result. Other things you can try are:

• Increase oven temperature: Raise the oven temperature by 15–25° F.
• Add more liquid: Add 1–2 tablespoons of liquid for every 3,000 ft of elevation, plus an additional 1.5 teaspoons for every additional 1,000 ft. You can also use extra-large eggs as extra liquid