My favorites for years - deerskin chopper mitts with REI inner mittens. I get the largest chopper mitts I can (XL or XXL). Treat the leather once or twice a year with SnowSeal or Obenauf's Heavy Duty LP. The combination is comfortable and warm down to 30F or colder. As I warm up, I often pull the inner mitts out and just use the leather outers up to 70F plus.

I don't know how mittens play with brifters. Shifting DTs is easy. I lose some hand positions but gain some others because the leather (properly treated) is so soft and grippy that handholds that would be very tiring with regular gloves are easy and relaxing with the mitts. Another plus - they are easy to get on and off so riding along and getting into jersey pockets and opening wrappers is not hard. And after a mechanical leading to very cold hands from handling cold aluminum, fingers together warm like isolated glove fingers don't.

Google chopper mitts. As far as I can see, there is one or two companies making them and many marketing the same mitts for radically different prices.

Ben