I'm not sure about mittens but Hestra makes top notch gloves. They also have mittens so I'd assume they are built well also. http://www.hestragloves.com/#/us/
These Dakine Mustang Trigger mitts are a couple years old, but if you could find a pair online or something they'd be epic. I've ridden with the glove version for 4 season, and wouldn't use any other glove.