I have used a few different methods eg fill a pyrex jug with boiling water. cut the butter you want to soften into small pieces and arrange them into a stack which will fit inside the inverted pyrex jug. Empty the jug of its water and invert it over the stack of butter pieces, and leave for about 20 minutes. The warmth of the atmosphere inside the jug will soften the butter.
An alternative method is to fill a mixing bowl with room temperature clean water and then float pieces of butter in it. Then scoop out the butter and use it in your recipe. You might end up with a few drops of water in your recipe too.