Sometimes, you just need cake. I've baked this from scratch before, but this is the one time I'm going to advocate boxed over homemade: when you're deep in the backcountry it's just too much fuss to pack in all the things you need to make a decent cake.
The last time I made this, I was morel picking between Liard River, B.C. and Barney Lake, in the Yukon Territory, with fellow adventurers/friends “Long” John Sutherland and Joslyn “Butch” Kilborn.
It was Long John's birthday and one of our friends back in main camp thought of him and sent us up everything we needed to make the cake by river boat.
1 boxed cake mix
1 can of frosting
2 tsp baking powder
1 can of beer
Start by making a good, hot fire; put some big wood on there, because you're going to want lots of coals. When it's good and roaring, let it burn down.
In a bowl, mix the egg, cake mix and beer together* vigorously until there are no lumps and the batter is thick and smooth.
When the coals are ready – when they are glowing and thick, that is – add the baking powder to the batter** and whisk thoroughly.
Pour the batter into a well-greased pair of cast iron skillets, cake pans, or other fire-proof, heat-tolerant vessel. A dutch oven works the best, but other substitutes produce similar results.
Cover with lid or triple wrap with tin foil. Very carefully, with a shovel, dig out a hole in the coals. Don't dig down to the dirt, but lay the cake in the fire pit so that it has coals below and all around it, like an egg in a nest.
Using the shovel, cover with coals. Resist the urge to add more wood – it will just scorch the outside.
Leave for 25-30 minutes. Uncover the cake with the shovel (carefully) and check with a toothpick. If it comes out clean, the cake is ready – it comes out wet and sticky with batter, put it back and cover it with coals for another 10 minutes. Maintaining an exact temperature is difficult with campfire cooking, so cooking times may vary.
When cake is finished remove (carefully!) from the coals, take off lid or tin foil, and let cool before attempting to remove from vessel.
Turn the cooled cake(s) over onto a plate and inspect. If there are any places that have been burned (sometimes that happens, don't sweat it) carefully cut/scrape off with a knife.
Cover with delicious, delicious icing. Eat immediately, as cake is highly perishable in these conditions (as well as very attractive to bears and difficult to store safely.
*if you have milk or water, you can use it, but beer works very well and is non-perishable.
** if you don't have baking powder, don't worry about it – it just gives the cake a little extra boost in these uncertain conditions and helps stabilize the rise, especially if the cake mix is old.