I have something very painful to admit, but this time it doesn't require penicillin. Brace yourself: I used to wear moon boots. To those of you who don't know what those are, I have one thing to say: get off my lawn.
Those of us experienced in the wearing of moon boots in arctic conditions know one thing: there is no way in hell those things kept astronauts' feet warm on the moon. We were deceived! Luckily we have more options these days, and at the top of that list should be Salomon's Tundra boot. Here's the skinny: mid/high length, nice furry cuff, basic reliable lug lacing system, waterproof, binding-ready with rubber nubs on the heel and oh-so-light at 3lb/pair. Fit is snug, so go up a half-size.
-40F? At 3lb? True, true. The secret: Salomon's evil engineers went to Disneyland, captured Tinkerbell and forced her into a captive breeding program to produce hordes of fairies. They then ground these little bastards up, branded the resulting mixture as Aerogel and insulated the boot with it. Result: I had no problem comfortably trekking about at -20F with only a thin liner sock, and I know it can go colder. Thanks Tink!