Yes, it's generally safe for butter to sit out overnight, especially salted butter, which lasts longer due to its preservative properties, but it's best to keep it in a covered dish away from heat and light, as it can eventually go rancid or develop off-flavors, particularly if your kitchen is warm (above 70°F/21°C) or if it's unsalted or whipped butter, which should be refrigerated.
Butter also consists of at least 80% fat, and bacteria needs water to grow. Since butter is mostly fat, it creates a barrier that makes it almost impenetrable to bacteria. Another factor determining how long butter is safe to be left on the counter is the presence of salt.
It should be fine. Salted butter will last longer out of the fridge than unsalted though. Some people leave butter out all the time.
To soften butter, we recommend leaving it on the kitchen counter for 30 minutes to an hour. Keep in mind that the speed at which your butter will soften depends on the temperature of your kitchen.
Best to keep it at room temp. I keep my butter out once I remove the wrapper. I would leave it at room temp unless you rarely use butter. It stays safe for quite a while and if it is not good anymore, you will know immediately because it smells wrong.
European homes often leave butter out on the counter. This makes butter soft and easy to spread. People do this because it is a tradition. European butter has more butterfat, so it stays fresh longer.
While butter is less likely to spoil, that doesn't mean it can't go bad. Left at room temperature for days, butter can oxidize and become rancid.
Butter can absolutely be kept out on the counter, but only under the right conditions. If you're storing salted butter, a couple of days at room temperature is usually safe. Unsalted butter is best left out only briefly, and garlic herb butter should always be refrigerated after two hours.
Salted butter is less prone to going bad on the counter than unsalted butter. If you're a serious supporter of leaving butter out, go with the salted kind. It's important to note, however, that the amount of salt in different brands of butter can vary.
Butter will usually bear an expiration date on its packaging, but remember: use-by, best-by, and sell-by dates are indicators of quality, not food safety. So while it's best practice to consume your butter before you reach those dates, it's generally safe to go beyond them, as long as you refrigerate or freeze it.
You will notice that French families often choose to store butter at room temperature, especially salted butter. Salt helps protect the butter and keeps it from spoiling.
“Do not leave butter at room temperature for more than 4 hours,” says Engen.
Don't refreeze butter that's already been frozen and thawed, as this can affect both texture and taste.
Eating rancid butter is unlikely to lead to serious illness, but it might cause an upset stomach or mild digestive issues, such as nausea. If you suspect butter is spoiled, it's best to err on the side of caution and toss it.
Smell: If butter smells sour, cheesy, or otherwise off, discard. Taste: If you notice a sour, bitter, or off flavor, discard. If the butter simply absorbed flavors from nearby garlic or onions in the refrigerator, it's still safe to use in cooking (although you might want to avoid using it for baking applications).
Sour or rancid smell: Fresh butter should smell clean, sweet, and creamy. Any sour, fishy, or soapy odors mean spoilage.