Zen is what turned Shakyamuni into a Buddha. He woke up when he saw the morning star after all night meditation. "Ah!"
Therefore, it's clear that "Buddhism" comes out of Zen. In fact there's no such thing as Buddhism apart from the speculations of academic eggheads -- in Asia, they just say "the Buddha Dharma."
The essential Buddha Dharma is the Dharma of Sudden Awakening. Shakyamuni wordlessly transmitted it to Mahakasyapa on Vulture Peak, who woke up (experienced satori) when his Teacher smiled and held up a flower.
The direct Mind-to-Mind transmission continued through twenty-eight Indian meditation Masters before Bodhidharma brought it to China. The Chinese had their own Buddha named Lao-Tzu, but Zen still hit China like a thunderclap out of a clear blue sky or a big iron bell ringing in empty space.
Zen is the essence of the Buddha Dharma. In Tibet they call it Mahamudra and Dzogchen. There will always be people trying to cover it up with mind and intellect, words and letters, but Zen always eludes these people.
"Ah! Ah! Ah!"