The “buttery-ness” of some Chardonnays comes from new oak barrels.
When a wine is described as tasting buttery, that flavor came, not from the barrels, but from malolactic fermentation. What happens is simple: malic acid in the wine is converted to lactic acid. Why do we care? Because malic acid has a tart mouthfeel (it’s the crunchy acid in a crisp green apple). Lactic acid, on the other hand, has a mouthfeel that is much softer (it’s the leading acidity in milk). Thus, malolactic fermentation dramatically changes the way a wine feels on the palate. A wine that has undergone malolactic fermentation has a texture that’s creamier than the wine would have otherwise been. The story doesn’t stop there. During malolactic fermentation, a by-product called diacetyl is produced. Diacetyl is the molecule that makes butter taste buttery. So, wines that have experienced malolactic fermentation—like Chardonnay—are often taste buttery. As an aside, there are technique winemakers can use to lessen the buttery flavor (if they want to).