The Hestra Heli 3 Finger Mittens are some of the best mittens for backcountry skiing. They are lightweight, but have a liner that can be taken out to dry out. A thin liner glove can be worn with the mittens to make them warmer, and the having the index finger separate make the mittens more versatile than a full mitten. They also have an elastic wrist holder, so you can take the mittens off and they will stay attached to your wrists. The palms of the mitts are also leather, so they are more durable for climbing and rope work.
For really warm mittens that can be used for backcountry skiing we highly recommend the Outdoor Research Alti Mitts. They are a bit bulky, but are super warm for when temperatures are low or you need to warm your hands up quickly. They also have leather palms and removable inner mitten liners.
We also recommend the Outdoor Reasearch Baker mitten shells. These are one of the most versatile pieces equipment you can have in your backcountry skiing kit. They are easy to use over a pair of gloves and add an extra layer of warmth that keep your digits warm when temps dip. They are durable, lightweight, don't take up much room in a pack or inner pockets of a jacket, so you can easily stash them way when you don't need the extra warmth or protection.