1. Preparing the corn: Build a medium fire in a charcoal grill with the coals concentrated at the center or off to one side, or preheat a gas grill to medium. Peel back the husk of each ear of corn, trying to make as few tears in it as possible and keeping it attached to the ear at the bottom. Remove and discard the silk. Smear 1/2 tablespoon of the butter over each ear and then press the marjoram sprigs into the butter. Pull the husk back into place to completely cover the kernels. Tear off a piece of the husk about 1 inch wide and tie it around the end of the ear in a knot to keep the husk firmly closed.
2. Grilling: When the coals are ashed over and glowing or the gas grill is preheated, arrange the corn on the rack, away from the concentration of coals, and cover the grill. Cook for 10 to 20 minutes over the indirect heat, turning the ears from time to time. The cooking time depends on many factors, the heat of the grill, the tenderness of the corn itself, and your own preference. Test the corn by piercing a kernel with the tip of a paring knife, it should go in easily but still have some "pop" to it. As it grills, the husks will brown and blacken and they might split apart in places and expose the kernels, but the corn will be fine. If the husks catch on fire when you lift the lid to turn them, replace the lid right away.
3. Serving: The grilled ears look beautiful when you present them on a platter in their husks, but it can be a struggle to remove the husks at the table. You'll do your guests a favor if you bring the platter into the kitchen to unwrap the corn and remove the marjoram sprigs that flavored it. You can then line a platter with some of the browned husks and stack the hot ears on top, using the cooked marjoram to garnish. Allow your guests to sprinkle the corn with salt to their own taste, and pass lime wedges if you wish.
Herb Substitution: In place of marjoram, use an equal quantity of lemon thyme sprigs.