Something is wrong with my process. I have now encountered the same off-flavor in two consecutive batches, which I think is excess diacetyl. While I can't really say it tastes like butter or buttered popcorn to me (as described in various Internet sources) I can see how it might taste that way to others.
The last batch of Por Favor has been hard to take because of this flavor. I have been describing it as "yeasty" because my taste sensor isn't calibrated very well. Yesterday, I kegged the first carboy of the 100201 Geordie-Boy Ale and carbed it. Imagine my surprise when I took that first big sip and found the same flavor there as well.
I'm looking into what could be causing this. As usual, the HomeBrewTalk forum has a wealth of knowledge on the subject, and based on what I'm reading, it appears that the main factor contributing to excess diacetyl is a combination of lazy yeast and lack of patience. I confess to the lack of patience part. As far as the lazy yeast is concerned, I am thinking that my poor winter temperature control has to be to blame. Both these batches were made when I really didn't try to control the temperature more than what was available in the environment.
The forum post states that having a diacetyl rest at 75 F will help the yeast clean up their mess. I don't think I've been able to come close to 65 F on a consistent basis recently, much less 75 F. Thinking back, I don't recall having had this issue with earlier batches, but for them the fermentation temperatures were a lot closer to 68 F in the first place than I've been able to manage recently, so it's possible that the yeast did more cleanup as they went along and I didn't need a diacetyl rest.
Now I'm in a quandary. I have five gallons of Geordie-Boy still in a carboy but I moved it to the cooler to cold crash yesterday. Should I get it out and let it ride at ambient for another week? If I do that I need to find a warmer spot in the house. What to do, what to do?
I opted to move it carboy and placed it into the warmest room in the house. I'll leave it there for a week and see how it tastes.