Spell Master, Variant (5e Feat)
From D&D Wiki
Prerequisites: The ability to cast at least one spell
When you choose this feat, choose either acid, cold, fire, force, lightning, necrotic, radiant, or thunder damage. Whenever you cast a spell that deals damage of one of the aforementioned types, you can instead deal damage of the type chosen for this feat. You can take this feat multiple times, selecting a different type each time.