Just made this recipe, and turned out pretty well, except the porcini rub burned when I browned the the tenderloin; it was positively charred. I'm...

I suspect that the final temperature is the issue. I routinely remove beef of any type from the oven when it reaches an internal temperature of 118-120º, then allow it to rest for at least 15 minutes before slicing. Given that once meat passes an internal temp of 100º, the temperature begins to climb rapidly, so it needs consistent monitoring. An internal temp of 125º is going to yield not only a much crispier crust, but also a doneness closer to to medium at the center, and well done at the extremities.

I made this dish last night and the roast had been "curing" overnight. With your experience in mind, I browned the meat just enough to get a crust on it (not sure if the roast itself browned much at all). By the time I took it out of the oven (was larger than 3#, so it took bit longer...) the crust was pretty dark, but didn't appear burnt and tasted great! I wonder if it's a fat issue. The roast had a substantial fat cap that I left intact for flavor. It may have prevented the roast from burning... but I wonder what other, more experienced, readers might say?

I tried the rub out on some steaks before I made the tenderloin at Christmas (too nervous about ruining a large expensive cut of meat) and the first time, I charred the rub as well. What I made sure to do the second time was to not just "dust" the meat with the rub after coating the meat in oil, but to actually rub it into the meat, the nooks and crannies, and incorporate it well with the oil coating before browning. This seemed to work nicely to protect the rub. I have no idea if it was just a fluke though!