Butter needs to be melted or chilled for many recipes, but most baking recipes usually encourage cooks to bring their ingredients to room temperature before starting to mix things up. Everything incorporates much more easily when it is all at the same temperature and you generally get a better, not to mention a more consistent, result when you prep your ingredients this way every time. One of the good things about summer is that it doesn’t take very long for butter to come up to room temperature (about 70F or so). One of the bad things is that it is easy to over-soften the butter.
Is it possible to over-soften butter? Yes, it is. Over-softened butter is where the butter begins to melt. Butter doesn’t have one specific melting point because its composition varies with different percentages of butterfats and liquids, so instead it has a range of temperatures in which it can turn from a solid to a liquid. This is roughly 85-95F. Softened butter blends easily with sugar, creating a light and fluffy mixture where the sugar is suspended in butterfat. When the butter melts in the oven, you are left with little pockets of air that give a baked good a fine and tender crumb, as well as a little bit of a rise. Sugar cannot be suspended in melted butter, so no pockets of butterfat can form and a baked good will be a bit more dense than one made with creamed butter.
So, softened butter and melted butter will give you different results when you bake, even if your butter is only partially melted. It is best to stick with softened butter – and to check the butter frequently on hot days to make sure it doesn’t over-soften – when a recipe calls for it.