These are the Maine Morning Mitts by Clara Parkes. I've made about six or seven pairs of these so far and each recipient has been pleased with them. Thanks to Felicity for being a hand model!
The pattern is available free on Ravelry.
Any fairly bulky one-ply seems to be working well. Mods: knit the cuff for five inches before beginning the thumb gusset; pick up six sts around the top of the thumb hole; knit the thumb a little longer than suggested; [added: EZ Sewn Casting-off].