over-baking is the most probable cause.
baking temperatures and times always have to be taken with a grain of salt. oven thermostats can be wildly off, whether you bake high or low in the oven, what you bake on, etc. etc. will all affect baking time.
one clue in this recipe is where it says to bake until the sides are brown. at that point, nearly any cookie will end up being hard and crunchy instead of moist and chewy. that said, some cookies are meant to be crunchy.
the simple solution is to bake a couple of minutes less and/or at a slightly lower temperature.
other things to try:
- increase the fat content
- instead of rolling them out and using extra flour, just make little balls and flatten them with your hand on the sheet.
- sugar adds not only sweetness, but also moisture to cakes, muffins, etc. but i think in cookies, it may instead make them harden as the sugar caramelizes, so decreasing the sugar may help
- make the dough softer by beating in a little milk with the egg before adding.
- use an extra egg, (essentially adding more moisture and fat at the same time)
good luck next time around. send me a couple if they come out good