Buttons are fine. Definitely saute till dry, but don't go any further than that. I'm not so sure that you can't add butter at the beginning. Remember that the water will cook off at 212 degrees, but the butter won't. That's the reason sauce reductions intensify taste. Almost everything except the water stays. However, it probably wouldn't hurt to wait to add the butter after the water is gone except that once the water is gone you will have a true saute and you have to watch carefully or you can overcook them. The great thing about mushrooms is they absorb flavor. Add some kosher salt at the beginning and they have much more flavor. Add chicken stock and they're even better. Canned mushrooms are fine if you're in a hurry, but nothing beats freshly sauteed!