For this type (cf. Idiotikon I: 341), Kluge gives the following short entry:

"Anke(n), (replaced by Butter) Sm ‛Butter' per wobd. (8th century), mhd. anke, ahd. anko.
Although only German has preserved the word, g. *ankwōn m. ‛fat, butter' is to be assumed as a continuation of ig. (weur.) *ongwen- ‛ointment, fat, butter' (in various degrees of ablaut), cf. l. unguen n. ‛fat, ointment', air. imb ‛butter' (*ṇgwen-) to the verbal root ig. *ongw- ‛to anoint' in ai. anákti, l. unguere et al. Thus, originally ‛ointment, grease'." (Kluge, 47 under Anke(n))

On the one hand, a plausible connection is revealed here; on the other hand, however, an improbable historical conclusion is drawn from it: Kluge interprets the word as an isolated Indo-Germanic relict, although it would be much more logical to explain this south-west German (Alemannic) type as being derived from Latin-Romanic (cf. unguere).
The Latin base mentioned with the velar has indeed been displaced by the variant *ŭngĕre (REW 9069) in the immediately adjacent Romanic area, as indicated by the palatalisation of g in roh. (Surselvic) unscher, roh. ((Engadine) uondscher, ita. ungere and others (cf. HWdR, 971). In what is now French, however, cognates of lat. ŭnguĕre are predominant (cf. FEW 14, 36 f.). These include forms with a clear semantic reference to milk processing, such as ogner 'donner son lait | giving milk' (with change of conjugation class) and ogna 'quantité de lait que donne une vache en une fois | amount of milk a cow gives at one time'. Incidentally, the participle unctum has given rise to the well attested fur. term for BUTTER ont, lld. onto, vonto (cf. ron. unt) in the Romanic-speaking part of the VA area. The proposed borrowing from Latin Romatic is phonetically possible and semantically self-evident, considering the numerous other romanisms in this onomasiological field. With regard to the much wider distribution of the type butyru(m), it is also reasonable to see an older type in the terms derived from the verb variants ŭnguĕre, *ŭngĕre, which was later overwritten by butyru(m).