I had never heard of putting SOUR CREAM in cookies til I had a roommate that made Sugar Cookies to decorate. Her Cookies were soft and stayed soft. She told me it was the sour cream. This recipe isn't hers. I never got hers before we parted ways, but I did find one that has sour cream in it and I use it everytime we want to decorate cookies. The cookie is good enough to eat without the frosting too.
Click Below for the recipe.
Another trick to use with cookies. If they start to get hard put a slice of bread in the container with them. They will have softened up by the next day.