There is no correct way to use "cannot help but." There are two traditional and synonymous idioms, "cannot but [do something]" and "cannot help [doing something]." Those who say "cannot help but" are conflating the two (in much the same some people conflate "irrespective" and "regardless" to create the nonsensical "irregardless").
In an earlier posting of the same question (now closed), someone very helpfully gave a link to this Google Ngram, which helpfully demonstrates that while the use of "cannot but" and "cannot help" has become steadily less frequent since the 18th century, while "cannot help but" has grown slowly since the start of the 20th, use of each of the first two still comfortably outstrips the third.
That said, "cannot help but" has become common enough that many people won't even notice, but those who do may dismiss the speaker as poorly educated and/or not focused on the meaning of his words.
As for correct usage, either "I cannot but deplore ..." or "I cannot help deploring ... careless use of language" will suffice.