Cookies are little sponges - they absorb moisture from the atmosphere. On a hot, humid day they can turn from crispy-crunchy to soft and limp within hours. Most cookie recipes will work perfectly when baked in temperate zones from September through mid-June, no matter what the weather. Fortunately, the cooler months are when we feel the most compelled to bake. When your cookie dough mixture is dry, add an egg yolk instead of water.
Water makes a dough tough.
The fat content of an egg yolk will keep the dough tender and its emulsifying action will help keep it moist after baking. No matter what the season be a watchful baker. Baking times when making cookies can vary, according to the conditions of your oven. Flour, while sitting on your shelf absorbs moisture from the atmosphere. Some brands of butter contain excess water.