This cookie recipe has been tested thoroughly to get the optimal amount of brown sugar to ensure the cookies bake up moist without being overly sweet. These cookies stay moist for up to 3 days when stored in a well-sealed airtight container. Though you could reduce the amount of sugar further, I would not recommend it as your cookies might turn out tough and dry. That being said, you could experiment and see what balance of sweetness and moistness works best for you.