Sprinkle both sides of the venison heart with the coffee rub. Reserve a couple of pinches for the whiskey-butter sauce. You can cook on a camp stove or build a small fire and burn the wood down to coals. Place a cast iron over the medium-high burner or on top of a grate over the fire.
Once hot, add a tablespoon of oil and sear the heart on both sides until cooked through. You should treat it as you would a steak and serve with it still pink in the middle. The thin flap will only take about a minute or two per side, while the thicker steak could take up to 3-4 per side. Remove the heart to a cutting board and let it rest.
Remove the heart to a cutting board and let it rest. Deglaze the pan with the whiskey, scraping up fond (the cracklings and brown bits) at the bottom of the pan with your tongs. Let the alcohol boil off for a few seconds, then add the butter and a couple pinches of coffee rub to season.