Yeah -15 is quite insane, a small/portable rocketstove would do you wonders to have a coffee/tea or cook even, with nothing but twigs for fuel.
A great trick for staying warm and comfy is to place a decent cinder block sized rock/concrete block or brick around/in the fire and let them heat up thoroughly, at least half an hour, and then they will radiate quite a lot of heat for the first 2 hours (wrapping them in a dirty t shirt afterwards would suffice to handle them easier and not get burned) and will have quite enough heat 6-8 hours later to warm you up all through those long winter nights, you can sneak them next to your back or around you but especially at the feet as they regulate your body temperature and will circulate that warmth throughout ;D.