Updated version is available as an individual pattern from Cotton & Cloud. The new version is worked top-down seamless method.

Printed copy can be purchased from here.

ERRATA IN THE MAGAZINE:
If you are knitting size 12, please start the body on row 3 of chart A (same as size 8 and 10).

I have made a video tutorial (here) about how to knit a tidy and strong buttonhole using crochet cast-on method which may be useful for this pattern ;)