Build a campfire or preheat an outdoor charcoal grill; allow fire to burn until a bed of coals has accumulated. Rake coals into a flat bed on one side of the fire.

Meanwhile, combine brown sugar and cinnamon in a small bowl; spoon into apple core. Wrap apple in a large piece of heavy-duty aluminum foil, twisting extra foil into a tail for a handle.

Place foil-wrapped apple in the coals; cook until softened, 5 to 10 minutes. Remove apple from the coals; unwrap carefully to avoid being burnt by hot sugar.