Finally had a failure! (does that make me a real cook or something?).
Followed Alton Brown’s recipe. I had made it once, successfully. Had enough ingredients for a second batch so I tried that yesterday.
Got to the “put a lid on it for 3 minutes” step and realized I’d forgotten the corn syrup. Whoops. Quickly stirred that in, then did the lidded boil.
After that, I attached the thermometer and let it cook until 234-235 degrees. Oddly, when I tested drops of it in cold water, the drop didn’t seem all that homogeneous - more like specs of very dark, suspended in lighter syrup - but the thermometer and the drops all said “soft ball”. So I cooled it down - probably to about 170 (which was about where I’d cooled the previous batches) then into the KitchenAid. It looked normal enough - though there wasn’t the really thick layer stuck to the bottom of the saucepan like there was with the previous batch - I hardly had to scrape the bottom of the pan at all whereas with the previous batch, if I hadn’t scraped the bottom, there would hardly have been anything in the mixing bowl. The bit I tasted didn’t feel right in the mouth, either.
Beat, beat, beat… beat, beat, beat… beat, beat, beat… finally it got “matte” and when I tasted a bit it had that finely-grained mouthfeel.
Into the pan, a few hours of cooling, and I pressed gently on the top - too much give. Pulled it out by the foil lining (I line the pan with butter-covered foil, that way I can get the whole mess out onto a flat surface for easy slicing). Cut into pieces, ate one - tasted good but a bit softer than expected.
Figured this was one for the fridge so I popped it into the fridge. Tried a bit this morning - the pieces were no longer distinct. Though they had firmed up quite a bit. Oh well. I re-cut the squares (which were visible, just the cut no longer went through the bottom of the slab).
I guess I didn’t cook it quite long enough despite the thermometer - either that, or the lateish addition of the corn syrup really made a difference!