I can't tell you how many times I have wanted to bake only to realize the butter needed to be softened. Unless I am melting butter, I am not a fan of the microwave to soften butter. I either over do it, resulting in melted butter and/or butter with a melted hole in the middle or I have to change the position of the butter in the microwave every few seconds to make sure it doesn't melt which is really annoying.
This is a particularly vexing problem at the restaurant as I get in at 5:30 in the morning and nothing is room temperature. We have gas ovens so they are always warm. So what I came up with was slicing the 1 pound block of butter into thin slices, placing them on a plate in a single layer and putting them in the oven while I got the rest of the recipe together. It is a life saver. The butter can melt if you aren't careful but generally, if I don't forget it, the butter is perfect when I am ready for it.
I use the same technique at home but I don't put it in the oven. Since I am working with much smaller amounts at home, it is easy to slice it thinly, put it on a plate and let it sit at room temperature while I go about prepping the rest of the recipe. It works every time - unless your kitchen is really, really cold - like with no heat and it is zero outside.
There have been discussions about the perfect temperature for softened butter with 65 degrees coming to the front of my mind. I remember doing this and finding that the butter was too stiff and cold at this point and when creamed with sugar, the mixture was sandy. Having nothing more to do that day (that is sarcasm at its best) I started testing the temperatures for creaming butter. The result was, in my kitchen, 70 to 72 degrees seemed to cream the best. However there is an innate problem with this exercise, other than a huge waste of time. It totally depends upon the proportions of butter to sugar and visa versa. If there is a lot of butter to sugar, almost any temperature will do. However, if you reverse this and there is a lot of sugar and little butter, a sandy mixture results every time.
So my advice is to get the butter softened, but not runny or liquid. This technique seems to do the job if you forget or can't get it done ahead of time.