I brewed a honey kolsch (all grain kit from norther brewer) and I think I taste diacetyl... its a little creamy/buttery maybe. I did do a 90 minute boil. It fermented starting at 62 and I raised it to 64, finally letting it spend its last few days at 68 (wyeast 1056). I let it lager for 4 weeks. I tried it and it had the buttery/creamy taste. So I took out of the fridge and put it upstairs where it spent 5 days at 70 - 74 in hopes of cleaning it up. No luck, it still has that flavor.. not displeasing or rancid in anyway. Is this most likely diacetyl or could it be something else?
What about the aroma of the beer? Do you detect the same buttery note in the nose? If so, then it might be diacetyl. If you don't detect it in the nose then it could just be the way the malts you used are playing together along the american ale yeast.
Did you underpitch your wort? Did you aerate well? Usually a proper diacetyl rest is done in the high 60'sF (like you did), but allowed to happen before the yeast reach their terminal gravity and typically around the last third of the fermentation. The trick is to start the rest while the yeast are still fairly active so they can reabsorb the byproducts produced during fermentation (diacetyl, acetaldehyde, etc). Trying to perform a diacetyl rest after 4 wks of lagering is not going to help much as the yeast have been chilled to inactivity.