When I do felted mittens, I do the cuff in regular UN-FELTED ribbing. I makes the mitten fit better and allows for some variation in size if it is a gift. I used cotton yarn or string about the same size as the yarn I'm knitting with. I knit a few rows before starting the mitten. Then I change to the real yarn and knit the mitten. After they are felted, (of course the cotton yarn does not felt) I take out the string and pick up stitches through the holes that are left by the non-felting yarn. Then I knit a regular ribbing. This whole thing takes longer to explain than to do. But it works!
Good luck with your mittens