With just a few exceptions, most baking recipes call for butter to be softened or at room temperature. Softened butter spreads easily onto bread and toast, and it whips up into a fluffy mixture with sugar that helps produce a fine, tender crumb in baked goods. Chilled butter is too hard to cream into sugar easily and melted butter simply begins to dissolve the sugar, and does not create the light mixture produced by creaming.
The best way to soften butter is by leaving it out on the counter for an hour or so before you will need to use it. The exact time will vary depending on the temperature in your kitchen and in hot weather your butter will soften much more quickly than in cold! Softened butter should yield easily to a gentle squeeze of the wrapper and it should have an easy-to-spread consistency, so that a butter knife can easily cut through it and scoop some up. The butter should not be so soft that it cannot hold its shape or that it has begun to melt. If it is very hot, keep a close eye on softening butter so that it does not over-soften and pop it back into the fridge before it gets to that point if you’re not ready to use it.
If you need to soften your butter quickly, it is not a good idea to pop it in the microwave for a few seconds. This can melt the butter and, while it will probably not have a big impact in a batch of chocolate chip cookies, it actually can impact the fluffiness of the crumb in a cake recipe. The best thing to do is to chop the butter up into small pieces, as they will soften faster than a whole block of butter. You can put the pieces into a mixing bowl and beat it (starting on low speed) with a mixer or the paddle attachment of a stand mixer to encourage it to soften. A few minutes of mixing along with 10-15 minutes of sitting out in small pieces should soften the butter enough to use in a recipe.