Cookies that stay chewy after cooling

First of all, always remember to slightly underbake (only a minute under lowest time). As soon as you remove the tray from the oven, give it a good WHACK! On the open oven door or other hard surface. This forces the air out of the centre of the cookie that would have made the cookie crispy after cooling, so you have a nice chewiness that lasts a couple of days (not that your cookies will!!)