So here are some sketchy instructions if you want to try with your favorite self patterning yarn!
This was an aran weight yarn and I knit the mitts with a 4.5mm (no. 7 USA) 20cm circular needle by Addi. I did start them with a longer circular using the magic loop method. For the gauge, I was aiming for a 7" circumference for my wrist and hand - that would probably be a lady's L or a man's S.
So here is my "recipe" - kind of sketchy but they really are simple to make!
2x2 rib for the cuff length you want - mine is about 2.5 - 3"
then switch to stockinette for about an inch.
Form thumb gusset:
row1: M1, K3, M1 knit the rest of the stitches
row 2: knit all stitches
repeat these two rows until you have increased a total of 8 stitches and you have a thumb gusset of 11 stitches.
Next row, P11, Knit to the end of the row
Next row, Knit all stitches
Thumb bind-off row: bind off the first 11 stitches, knit to the end of the row