It was actually margarine that was used in the movie scene. This was the main reason why actress Maria Schneider got so miffed - she was a purist - and in her desire for cinematic authenticity and realism, became incensed that director Bernardo Bertolucci would decide to so crassly substitute a cheap imitation to replace the real thing. In typical French haughtiness, she sniffed: "Phffff, I can't BELIEVE it's not BUTTER"!!!!🤣🤣🤣🤣🤣