There are several reasons that these cookies stay soft. They have a relatively high amount of moisture, with an extra egg. They also take a cue from our favorite chocolate chip cookie and dispense with the mixer. There is no creaming of butter and sugar, so there's very little air in the dough. Less air, less dryness, more moistness. Also, melted butter and no mixer means less work and less cleanup for you. Bonus!
The cookies also bake very fast at a high temperature, so, again, they have less chance to dry out. It's very important to underbake these a little; that will help them stay soft and chewy.
One more tip: For really fresh and special snickerdoodles, try using different sorts of cinnamon, like Vietnamese cassia. Try grinding it fresh, too — it's easy to blend cinnamon sticks into powder in a grinder or even in a blender, and the taste will surprise you. It's shockingly bright and vivid. If you have only had cinnamon from a jar, try something fresher and see if it doesn't inspire you.
1 cup (2 sticks) unsalted butter
1/2 cup dark brown sugar
1 cup white sugar
3 cups flour
1/4 teaspoon cinnamon
1/4 teaspoon nutmeg
1 teaspoon baking soda
1/2 teaspoon salt
1 teaspon vanilla extract
For the cinnamon sugar:
1/4 cup white sugar
1 tablespoon cinnamon
1/4 teaspoon nutmeg
Heat the oven to 425°F. Gently melt the butter in a saucepan or in the microwave and let it cool while you mix the dry ingredients. Stir together the sugars, flour, spices, baking soda, and salt. Whisk the eggs into the cooled butter and add the vanilla. Stir the wet ingredients into the dry ingredients, stirring just until it comes together.
In a soup plate or shallow bowl, mix together the white sugar, cinnamon, and nutmeg. Form small 1 1/2-inch balls of dough and roll them in the cinnamon sugar. Place them on an unlined, ungreased baking sheet and flatten slightly. Bake for 7 minutes then remove and let cool on the baking sheet for 5 minutes. Remove to a wire rack.
The dough can be refrigerated for up to 5 days, well-wrapped. It can also be frozen in logs.
(Images: Faith Durand)