My daughter just introduced me to "reuseable heat packs" https://www.amazon.com/HEAT-WAVE-Ins.../dp/B000E48LVM for example.
The cons are they are pricey, a little heavy and only last for an hour or so. The pluses are they generate a tremendous amount of heat in that hour. There is a clicker inside and once clicked the heat is nearly instant. A couple in your frozen boots in the morning, mitts or thrown in your sleeping bag before you got in would be a great a back up. You can re heat on the trail or when in town.
Best of luck and have a great hike.