Funny. When I first saw this thread, my first thoughts turned to a place to get alcohol. Don't have a go-to spot in the high-country in this regard. I'm kind of a beer guy, so it would be more like favorite pub anyway and my longtime favorite, Dixon's of South Lake Tahoe, changed hands, declined, then closed (at least I didn't see the original location when I drove by there after my Desolation "night run" back in September).
As far as bars that fit in the mouth, my go-to bar is the oatmeal raisin Clif Bar. 'Tis a bit sweet and chewy but it works when I'm up there and working hard. I will do the chocolately ones on occasion and I like them but I find the caffeine content is high enough that I if I consume one of those in the afternoon I have trouble sleeping later (morning is OK, though).
Speaking of fruitcake, a death march tradition of mine is to bake a loaf of banana nut bread (at home) and consume it over the first two days of the trip. Now the banana bread is much esteemed in our family even at sea level, let alone at altitude. This led to one "tragic" moment on a trip my wife and I took up the spectacular Lake Genevieve outlet slot canyon above Convict Canyon (this is a trip that eventually went over the crest to Ram Lakes--not really a true death march). We stopped to rest on a scree-covered sloping bench on the walls of the slot canyon and took out the loaf of banana bread to slice. It sort of plopped out and started rolling, escaped a desperate lunge, then vaulted into space... My wife and I nearly cried. I think we were imagining the taste of the banana bread we lost for the rest of that day.