I used to use a MTB version of barmitts made by Madden Mountaineering. They were great for keeping your hands warm, but I found that I was reluctant to pull my hands out to give hand signals, especially in traffic. It was awkward getting back in them. I did ride down to -4F/-20C using them, but I found that wool mittens in leather mitts were just as warm and removed that awkwardness. At those temperatures, you don't shift much, even downhill feels like uphill and just forget the wind, if you can. I would be very careful about where you use Barmitts. They are not for group rides, or heavy traffic.