Bar Mitts/Bike Pogies!
https://www.cyclingweekly.com/products/smitten-with-mittens-...
https://www.cyclingweekly.com/products/smitten-with-mittens-...
I'm wondering how they accommodate shifters and brakes but I'm sure they do since I saw them all over NYC last time I was there.
There are also lobster gloves where you pair two fingers next to each other to keep warm and still be able to use the breaks and shifters.
They make them in two warmth ratings, down to -15°F [1] and below -15°F [2]
[0]: https://jnyyz.wordpress.com/2022/01/11/dogwood-designs-pogie...
[1]: https://www.907bikes.com/product-page/dogwood-designs-winter...
[2]: https://www.907bikes.com/product-page/dogwood-designs-winter...