From my experience, the best way to use butter when baking is to let it soften to room temperature. However, sometimes one is short on time and must use the microwave to soften the butter. I always felt that cookies are better when the butter is a little firmer. Why is that? Is there a better way? Should I simply defrost the butter in the microwave carefully? Perhaps put it in a closed bag and run it under warm water?
To get your butter up to room temperature faster, you can slice it into very small pieces or grate it with a cheese grater and then leave it out in a single layer. It will get where you want it in around 10 minutes that way.
You cookies taste better with room temperature butter because they have more air in them. When you mix the butter, sugar and eggs in your cookie recipe you're creating an emulsion of oil, proteins and air. Those little air pockets in the cookie dough then help the cookie puff up when it bakes. When the butter is melted all the way before you mix it in, this emulsion has much less air in it than it should, so your cookies' texture isn't as good.
This article has a lot of interesting information about butter in cookies, including common mistakes people make.
The best way to get frozen or refrigerated butter ready for creaming is to cut it into chunks. (Never use a microwave: it will melt it, even though it will look solid.) When the butter is still cold, but takes the imprint of a finger when gently pressed, it is ready to be creamed.
cheat! use a faux butter. margarine is gross, unhealthy, and certainly not what i'm suggesting, but they have some nice faux butter out these days that have at least 30% real butter, and cream well direct out of the fridge. they have one here in NZ called olio i'm quite fond of, and i presume our food science tech isn't at an astronomically superior level compared with the rest of the world.