"Hot Butter" is an alias for the keyboard player "Stan Free". He's best known under his alias for his 1971 cover of the Moog synthpop instrumental "Popcorn", originally by Gershon Kingsley in 1969.
Stan was a New York-based jazz musician, composer, conductor and arranger. He recorded several albums (now out of print) under his own name with various combos. He also arranged and conducted for other many artists, notably the jazz vocalist Chris Connor (ChrisCraft), and the comedian Jack Carter. He died in August, 1995.