After the butter sits for a few hours the espresso powder will dissolve a bit more into the butter, turning it a mocha color. I didn't know this until after I took all the pictures.
You can keep the butter in the refrigerator, but it is best served at room temperature.
If you think that espresso flavor might be too strong, feel free to decrease the amount of powder to ½ tablespoon or use instant coffee granules instead.