It wouldn't necessarily "break off" anything. If the angular momentum of the post-impact planetary mass reaches or exceeds orbital velocity, and its temperatur

It wouldn't necessarily "break off" anything. If the angular momentum of the post-impact planetary mass reaches or exceeds orbital velocity, and its temperature is high enough, you get a novel type of celestial body - a synestia [1], a planet-sized hurricane of boiling rock. As the rock cools, magma rains inward re-fornming the Earth (Earth V2?) over decades, while everything far enough out (beyond the Roche limit [2]) would coalesce to form the Moon. 1. https://en.wikipedia.org/wiki/Synestia https://en.wikipedia.org/wiki/Synestia 2. https://en.wikipedia.org/wiki/Roche_limit https://en.wikipedia.org/wiki/Roche_limit