Go to a motorcycle shop and buy good leather gloves with an insulated liner in them. They aren't cheap, but they will help keep your hands warmer than regular leather gloves.
If you aren't worried about dextarity, go to a snowmachine dealer and get the best leather snowmachine mitts you can afford. My hands stay warm even on long snowmachine rides.
Treat them with leather treatment (like Dubbin) once or twice a season to keep them soft and supple.