Yarn bowls are my absolute favourite to make, I enjoy making each one different depending on my mood.
I use stonewear clay, throw a little thicker and cut just before leather hard. I wrap the top in plastic and dry upside down. Sometimes I will use paper wedges to support the swirl part if I notice some sagging (the wedges just burn up in the kiln)
after I bisque I sand the swirl as I once used a yarn bowl that didn't have a perfectly smooth swirl and it snagged my yarn and really annoyed me while I was knitting lol so now i'm mindful of how smooth the swirl and inner bowl is. I use a small sharp blade and hand carve all of my cuts, which is why I cut just before leather hard, I find that while there is risk of sagging for me its easier to carve when the clay isn't so stiff. Yarn bowls are my biggest seller. I personally don't like, wont use and wont sell a warped bowl but that's just me being picky. Slow drying, a gentle hand when carving and paper wedges for support helps me avoid warping.
Here are a bunch of bowls i've made.