Eventually, after a few days at room temperature, even salted butter can turn rancid, ruining its taste. If you are going to leave butter out, store it in an air-tight container, which can help preserve it longer and limit contamination with dust or other airborne particles. Also be careful to avoid cross-contamination with other, more perishable foods.
If you want to keep butter on the counter, Kandora says, it’s best to do so in small batches – just enough to use in a few days. Keep the rest in the fridge until you need it.