I've noticed that, all of a sudden, my chocolate chip cookies are spreading more than usual.
I was wondering if the (organic) butter that I'm using could be affected by the season.
I don't know about weather, but non-factory produced milk changes with the season because of the feed, which will affect your butter.
Over the winter, the cows are fed hay or corn, while over the summer, they're pastured, so get fresh grass.
In the Netherlands, there are designations such as 'graskaas' (literally, 'grass chesse'), which is made from the milk when the cows are first released to pasture in the spring. I know there is a designation for winter vs. summer cheese as well, but it's not coming back to me.
This may actually have to do more with the temperature and climate in your home than with the ingredients in your cookies. If you're making cookie dough while your heater is on in the kitchen along with a preheating oven it may just be warmer in there than it would be in the summer with the AC running, or other active steps being taken to keep the house cooler.
Also, if you live in a climate that is more humid in winter (or if your heating system includes a humidifier) it may make your dough more moist and make it behave a bit differently.
If you find your cookies are spreading more than you like, put the dough in the fridge for 10-20 min before baking so the dough firms up. The outside of the cookie will cook and stabilize before the cookie spreads too much, giving you a taller, thicker cookie.
If all your cookies spread, I would say adding an egg white should help, but I wonder if you are plopping your dough onto a warm surface. That will cause them to spread more as well. You might try backing off your heat just the slightest bit. Have you changed cookie sheets recently? Dark metal vs new aluminum or stone, perhaps airbake sheets? All of these things can cause you to need to tweek or vary your temp. and cooking time. The more butter you use in the recipe the crispier and flatter your cookies will be.