Many baking recipes call for "softened" butter, or room temperature butter.
The reason for using softened butter is that you want to beat the butter (also called "creaming the butter"), either with sugar or without, and it's very hard to beat a cold stick of butter.
Why beat the butter? To get air bubbles into it which will expand when baking.
The best thing to do to soften your butter is to put it out on the kitchen counter for a half hour before you have to use it. But sometimes we don't think that far ahead.
One popular solution is to put the cold butter in the microwave for 10 to 20 seconds. The problem with microwaving butter is that the microwave can heat the butter unevenly, and tends to over-soften or even melt the butter in places.
A New York Times article on butter in baking says, "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."
You want pliable butter for beating, not almost melting butter.
Here are a few tricks to quickly soften butter:
- Grate the cold butter using the large holes of a cheese grater.
- Cut the butter into smaller cubes, they'll soften more quickly that way
- Put the stick of butter between two pieces of wax paper and roll out with a rolling pin
See below for how to soften butter quickly using some wax paper and a rolling pin. This trick was taught to me several years ago by my neighbor, pastry chef Evie Lieb, who told me that she learned about it from a collective of Bay Area bakers called the "Baker's Dozen".
Tip: How to Soften Butter Quickly
Put the stick of butter between two large pieces of wax paper
Using a rolling pin, press down on the butter
Roll it out they way you would roll out a pie crust
When the butter is about 1/8 to 1/4-inch thick, lift off the wax paper and peel away the butter
(before it gets too soft to peel).
Voilà! Softened butter, ready for beating.