What makes a cookie soft? How can I make a sugar cookie softer?

Like you, I use only "real" butter. Since you're already doing that, here are some additional tips:

Most important: make sure you don't overcook them - they continue cooking a little on the pan even after you take them out. I remove my cookies from the oven when they still look slightly underdone in the middle.

Try adding a little cream of tartar - maybe 1/4 tsp

Try sifting your flour before measuring, or better, measure by weight instead of volume. (You could be getting variances in the amount of flour you use each time - so you are using relatively less / more butter)

I don't think the mixing method makes much difference. You could try using less eggs, but again, not sure that would make a big difference.

do not over cook the cookies. remove them from the oven 3-4 minutes before their actual cooking time. do not over mix the cookie batter. after mixing the flour and and the egg and milk, simply fold in the rest of the ingredients (nuts and chocolate chips) use margarine. do not skip on eggs or shortening.you can make it extra special by adding white or dark chocolate chips, chopped nuts or chopped dried fruits.Try searching yeolesweets on the web.

They have cookie choices/variations for inspiration.

Baking the cookies for a little less time would keep more moisture in the cookies. This would make the cookies softer.

Add crisco for half of the fat solids. Other half butter. Makes for a softer and moister cookie.

Use crisco or butter flavored crisco in place of margarine or butter in any cookie recipe. Your cookies will soft every single time no matter what.

One thing you can do instead of adding more ingredients is to take them out a couple of minutes earlier, that's what I do. When they look done but are only a little gooey, they get stiffer after awhile but are nice and soft! :)

with any cookie the more butter you use the softer your cookies will be

use more sugar and less butter or margarine, use oil instead.

Roll them out a bit thicker, that should help.

s.s. is right . it is in the butter and use a little bit more