For buttonholes that will experience stress (such as my pants waistband), some buttonhole feet have a single tooth at the top and bottom. Thread can be wrapped around the back tooth, running under the foot, and crossed in the front tooth. The satin stitch for each bar of the button hole covers the thread, producing a firm, almost puffed appearance.

I've also in a few cases doubled the thread in the single needle to produce the thickest satin stitch possible - I've used this for children's and men's coats.

If you have access to silk thread, it makes truly beautiful fine buttonholes and wears really well. Learn from my experience however, and don't use embroidery thread!

Looks very custom. I especially admire that collar; from personal experience I know material that thick isn't easy to handle in tight corners.