The trick to softening butter is to retain the integrity of the butter while heating it to a workable temperature. You absolutely do not want to melt the butter as you heat it.
(1) Place necessary amount of frozen or refrigerated butter on a microwave-safe dish in its wrapper, and set microwave to 50-75%
(2) Use short bursts of 10-15 seconds for frozen butter and 5 seconds for refrigerated butter. Turn butter after each burst.
(3) Heat butter until it can be depressed with your fingers but not yet liquefied.