No peeking! We all want to see what’s going on in there, but as soon as you open the lid steam escapes, the temperature drops and you can add an extra 20 minutes to your cooking time.
If you feel as though your butter is still too watery after puréeing, put it back in the slow cooker (or even a saucepot over medium-low heat) with the lid open for an extra 15–20 minutes, stirring occasionally until thickened.