Unfortunately butter melts and separates. Best to make ghee (clarified butter), removing the milk solids (which will also greatly increase the calorie count!). Carry it in a plastic jar and add to breakfast and dinner. It's easy to make, or evidently (per the second link below) you can buy it. It doesn't taste quite as buttery, but doesn't completely lose the butter flavor.

how to clarify butter

trail cooking with ghee

Sarbar's trail cooking.com site is mostly down as it's being reconstructed. In the past she has recommended using ghee, which is why Glenn's post made me remember this item!

Glenn's mention of trail mix (with lotsa nuts, not just peanuts) is a great idea! Nuts contain protein and, especially, healthy fats. They are my protein choice for "lunch" (which for me is an all-day snack).