There are some pictures online of similar growth in Rebutia muscula
. I don't know if those are some kind of hybrids, or just a different variety than the more common one. It's also possible that the stem will fill out and broaden during the summer. Perhaps, when full watering is started again, it will swell up with water storage.
I still wouldn't advise cutting back the stem, though. With cacti, when you cut a stem off, the top will never grow back. It will always be a dried cut end. I've done it in order to use the top to start a new cactus, but only when I was ready to throw the old one away ... either because it was dying, or got too large for the space I could allot to it, since my house is small.
If you start a new cactus from the cut off top, it will have to grow and mature before it can produce flowers. That will take some time. I would wait and see what your little "witch's hat" does during the summer growing season, before taking any extreme measures with it.
I would definitely
be interested in seeing how things progress with this cactus, if you wouldn't mind posting occasional pictures of it.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams