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Michelle Phan's built a YouTube empire of 6.6 million followers ... but she did it by jacking huge hits from Kaskade and Deadmau5 and slapping them on her hugely popular videos ... according to a new lawsuit.

Phan -- known as the "beauty bestie" -- is famous for the makeup tips she's been posting since 2007 on YouTube -- but Ultra Records says she's been using music from some of its huge artists without permission. Ultra is home to EDM giants Deadmau5, DJ Kaskade, and Calvin Harris, and many more.

According to the suit filed this week in Los Angeles ... the offending Phan videos have racked up more than 150 MILLION views. Ultra wants Phan to reveal how much cash she's made off her YouTube channel ... and it wants ALL of her profits.