Thank you ladies! I ended up using the "buttonhole stitch" as described in several of my books (Teach Yourself Visually Knitting, The Knitter's Book of Finishing Techniques, and Knitter's Companion).
It is stated to "reinforce" buttonholes, but also make them a tad bit smaller. I believe it's called "blanket stitch" when used on an edge.
The funny thing is, it's a suture pattern used to close skin - called the Ford Interlocking. I recognized it as soon as I started sewing. It's a little easier on animal skin than on knitting but it did the trick!
My buttonholes are just a bit smaller and stiffer, and now my cardigan doesn't want to open up.