This recipe calls for "softened butter" and says use any Google-able method. I have found this website to offer multiple useful ways:
* https://www.jessicagavin.com/how-to-soften-butter/
I however, use a variation on the "microwave it" method, which is: start with 1/2 sticks of butter (4 tbs), leave the butter in the wrapper and microwave it on high for 4 seconds. Let the butter rest some random amount of time while you tend to other things in the kitchen. Then flip it and microwave it again on high for another 2-4 seconds. After this, it's usually pretty soft with very little fully melted parts.