For a ¼ stick or smaller portion of butter, fill a microwave- safe coffee mug about ⅔ with water. Heat it until the water is almost boiling. Remove the coffee mug (with a pot holder). Pour the water out. Invert the empty, hot coffee mug over the butter and let sit for 1-2 minutes or until the butter is as soft as you want it to be. You can lift the mug along the way to check the progress.
Choose a coffee mug with a diameter that is larger than the piece of butter that you want to soften. You don’t want the sides of the mug to touch the butter.
For a stick of butter, use a rectangular stone ramekin or Pyrex dish that is large enough to avoid the sides touching the butter. Voila, you have soft butter!