Molten Impact is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This sorcery deals 4 damage to target creature or planeswalker. If excess damage was dealt this way, note that excess damage, then you get a one-time boon with "When you cast an instant or sorcery spell, this boon deals damage equal to the noted number to target creature or planeswalker an opponent controls."