The image above is the wrapper for an old spool of gimp, "vergolina" being the Italian word for gimp, and La Milanese being the brand name. I am guessing that this is how the French came to know gimp as "Milanaise", and thus the Milanese buttonhole would have found its name. Conjecture, but likely.
I've discussed this buttonhole a few times, mainly because I didn't know how to do it and that was driving me crazy. The example above is from a coat by Smalto and I had a good look under a magnifying glass and finally figured that stitch out- it is a fairly simple figure-of-eight stitch with no knot or purl. More on that soon. A reader was kind enough to reach out with another method of making them.
Matthew Reed did the CAP Tailleur Homme in Paris (a training program in tailoring) and did his internship at Cifonelli, a famous Parisian house. (If you read French you can find a recent interview with Lorenzo Cifonelli here) Matthew was kind enough to photograph a high-contrast buttonhole sample that he learned at Cifonelli, and the steps involved in making it- you will notice that in this version there is a knot underneath the gimp, which I rather like because it raises the buttonhole higher off the surface and makes it a bit more pronounced (subtletly be damned). So thank you, Matthew.
The gimp has been knotted and inserted between the layers of cloth- it must be lifted out of the way when taking the stitch.
Wrap the tail of the twist around the head of the needle.
I've been mumbling about buying a macro lense since my close-ups leave much to be desired, but seeing Matthew's photos I'm leaning toward a point-and-shoot instead, which would be cheaper and more versatile. So double inspiration, thanks to Matthew.
Franca made some interesting observations in the comments section. She has discovered what many have found, namely that many old tailors guard their secrets jealously... She compares the buttonhole above to the buttonholes she sees in her region (Abruzzo) but the buttonhole above has a knot, and the ones made in Abruzzo, particularly Brioni, do not have a knot- it is slightly simpler in certain respects, but require more precision. She has discovered that cloth that frays easily is not suitable, or at least will be difficult to make a stitch that is small enough even though she has overcast before stitching. I will suggest running a machine stitch 1/16" away from the cut location on each side (before cutting) which will help stabilize the area. As for the stitch itself, once I get my macro photography sorted out I will post a series of photos like the ones Matthew submitted, showing the Asola Lucida, al modo di Brioni. And yes, Franca, the only thing to do is to practice and practice some more.