Can't smell it anymore.......that's cause you have gotten use to it :B
Luckily you have metal duct work.......that means you have a good chance of being able to clean it out.
Fish a wire from that duct to the next register.....start off pulling dry rags thru then repeat each pass with a warmer rag and warmer rag (dry heat....maybe heated in microwave.......or with blow drier/heat gun).
Wet rags will not pick it up..just smear it.
And pull them from the other register to/out the register butter went into.....not the other way.
