According to OED, elfin is the correct form of the adjective meaning "of elves", as oaken is "of oaks", and twiggen is of "twigs." However, J.R.R. Tolkien's Lord of the Rings popularized elven which is now standard in American English.
Driving home from Grand Rapids last weekend, I ended up behind a Keebler truck with the motto "... a little elfin magic goes a long way." stamped across its back. This unfortunate usage of elfin, though grammatically correct, is cognate with effin. Intentional? Perhaps. One wonders how else this slipped past the marketing department.
It's always pleasant to encounter linguistic oddities, especially on a grey winter day of an exhausting weekend. Ah, words... a little effin magic goes a long way.