This was a very fine post and helped me with my decision.It's safe. But I wouldn't recommend it for three reasons:
a. It might give her the runs or upset her tummy.
b. It might displace real food or add extra calories she may not need
c. Cats are creatures of habit. Even if you stopped this practice tomorrow, she may ask, demand, or even steal for it for the rest of her life depending on just how much she enjoys it. And she's not going to know the difference between butter she can have and say margarine, mayo, or cream cheese that she probably should not have. You have probably created a cat who will hunt butter knives the rest of her days.
My last cat, Krista, the one in the avatar there, I used to let her lick the scrambled eggs bowl clean after I poured the eggs in the pan. I only did this for maybe a few weeks until it seemed like it wasn't really agreeing with her. Nevertheless, for the last few years of her life, every time I scrambled eggs, she would come running into the kitchen and yell at me for her share. I wish I could have kept giving it to her for how much she enjoyed it. But she developed IBD (unrelated to this) and egg was one of her no-go foods after that.
Briefly, she was on Pepcid. That had to be given 30 minutes prior to her meals. So she'd get a shot of liquid famotidine and I would set the microwave timer. When the microwave timer went off, I could serve her meal. She was only on Pepcid for maybe a month. But for the rest of her days, whenever that microwave timer went off, she would come running into the kitchen expecting a meal. It got to the point that I would supervise the microwave timer and stop it before the clock ran out.
In other words, her butter habit is probably safe, but also probably not recommended. I like to keep human and cat food separate. In the rare case where there might be crossover (a piece of meat), I make sure to put it on her plate rather than let her eat off mine. My current cat, Betty, is much better at not pestering me for my food because I never share my food directly with her.