It couldn't be cocaine or snuff because these were both new world plants that would not have made it to China by the 13th century, right? Is this just a gaffe on the part of the producers or is there some other kind of drug that Mongol emperors might have taken nasally?
I was actually wondering this when I watched the episode too. I did some research online, based on what was seen in the episode, and came to the conclusion it was likely ephedrine. In particular, the ma huang (Ephedra sinica) plant which is native to China. It's noted as the "world's oldest medicine" and has been used in traditional Chinese medicine for thousands of years. It's used as a decongestant to treat symptoms of asthma, colds, or influenza; but, it also functions (and what Kublai appeared to use it in the episode) as a stimulant. Snorting it would be a way for the body to quickly and easily absorb the ephedrine.
A few sources I found are here and here. I also don't know if my post passes the standards for an answer on r/AskHistorians. I just thought I would pass along what I discovered for myself. If anyone has more insight, or wants to correct me if I'm wrong, please feel free to do so. After all, I am sure Kublai had access to wide variety of plants from the Silk Road.