Butyric acid, butyrates, butyric acid esters… these are what you are describing…
the series is methyl (1 carbon), ethyl (2 carbon), propyl (3 carbon), butyl (4 carbon)… butter/buttery notes are typically 4 carbon/butyl … as the carbon chain gets longer, the aroma becomes more animalic, zoo-ish… but the esters (or “salts”) of these longer chains often have sweet, fruity notes that are the basis of most delightful ripe plums, apples, pineapples, and also are modified to make perfumes…