Keeping the bottom of your legs warm in very-cold-weather biking
I would suggest shoe covers, which look like a boot and cover the ankle.

Or wear mid-calf or knee length socks and roll the upper part down for double or triple coverage on the ankles.

Or, mini ankle warmers :-). 

Pics of my Pearl Izumi MTB overboots. The sole is exposed so walking is possible. (Not a product recommendation, just what I happen to own.)