My kids (and I) love snickerdoodles. I have a tried and true recipe, and they taste great. The problem is that once they cool completely, they are hard as rocks. We zap them in the microwave for a few seconds & they are great, but I want to know WHY they get so hard and how to avoid this problem. I store them in ziploc bag (for the short time they last around the house), but they seem to get hard almost immediately. What am I missing?
several things to consider...
how is your fat to sugar ratio? you might consider increasing your fat to retain more moisture. a lesser ration, meaning more sugar to butter will produce a flatter, crisper cookie.
are you using all white sugar? if so, you might try subbing in some brown sugar. if you're already using some brown, increase that and decrease the white by some accordingly. brown sugar produces a chewier, softer cookie.
you can also try pulling the cookies out of the oven early, say by five minutes, depending how long you're cooking them, then allow them to sit on the cookie sheet. they'll continue to cook, but i find they stay softer longer.
storage tricks: try putting a piece of bread into the storage container - the bread will get hard, and the cookies will stay softer. some also store in plastic ziploc bags... i usually never have a problem with the tupperware containers, but you might try ziploc first.
maybe report back on your recipe, so we can give you more focused feedback...
My first thought would be you left them in the oven too long. They need to be slightly underdone when you take them out (they continue cooking), cool on the tray a couple of minutes and remove to a cookie rack. If they're fully cooked coming out of the oven, they'll be hard when they cool. If they are hard the next day, add a piece of bread to the container and it'll soften them, as Emme recommended.
Get them onto a cooling rack right away - if you line the pan with parchment you can remove the whole sheet to the rack and not have to remove soft cookies with a spatula.
You can try melting the butter... this trick works for my choc chip cookies. Got the technique from Cook's Illustrated and the cookies stay soft when stored at room temp in an airtight container for a couple of days. Using brown sugar helps as well.