At home: Combine butter, zest, chili powder, salt, pepper, and garlic in a small resealable plastic bag. Mush around to combine thoroughly. Chill until ready to use, then bring to room temperature.
At home or campsite: Pull back husk from each ear without detaching from bottom of cob. Remove as much silk as possible and fold husk back over ears. Soak ears in water for at least 30 minutes and up to 1 hour.
At home or campsite: Drain ears, open husks, dry ears with paper towels, and spread evenly with butter mixture. Fold husks back over ears and tie in place with kitchen string or strips of husk. Corn can be kept at this point, chilled, up to 24 hours.
At campsite: Prepare grill for indirect medium heat (350° to 450°; you can hold your hand 5 in. above cooking grate only 5 to 7 seconds). If using charcoal, bank coals evenly on opposite sides of the firegrate, leaving a cooler center section; if using gas, turn all burners to high, close lid, and heat 10 minutes. Then turn off one burner and lower others to medium. Grill corn over cooler spot, covered, until tender and charred, 20 minutes. Serve with salt for sprinkling.
Note: Nutritional analysis is per serving.