I recommend trying Bar Mitts, from barmitts.com, as seen here:

http://mark.stosberg.com/bike/2010/12/emundo-vs-bakfiets-for-winter-kid-and-cargo-cycling.html

They add significant warmth and wind blocking. I’m using them here at 15 to 20F and 20 MPH speeds wearing a windstopper glove that I would usually rate only down to 25F— not a winter riding glove. For really cold temps wool mittens, etc could be worn inside of them. (It just hasn’t gotten much colder than that yet.)

The mountain bike style still has some room for improvement– there is still a gap for some air to flow in where the wires come in. If you are leaving the Bar Mitts on for the season, you could probably stuff part of an old wool sock down in them to solve it.

Some other minor drawbacks: The bell is inoperable inside the mitt. and my throttle sometimes sticks in the “on” position due to the tight fit, and needs to be manually pulled back. There’s also the limited angle in which you can grab or let go over the bars from, which usually isn’t a problem. Overall I rate them a “win” after a couple weeks of use.

It would also be possible to create an improved custom design if you could get your hands on some neoprene and someone with an industrial sewing machine. (Shoe repair shops tend to have these).