Lightly rub corn with oil. Lay corn on a barbecue grill over a solid bed of medium-hot coals or medium-high heat on a gas grill (you can hold your hand at grill level only 3 to 4 seconds); close lid on gas grill. Cook, turning often, until corn kernels are lightly browned all over, with some blackened spots, 5 to 7 minutes (see notes). Let stand until cool enough to handle, then, holding corn upright in a deep bowl, cut kernels from ears with a sharp knife.
Meanwhile, in a 5- to 6-quart pan over high heat, bring 3 quarts water to a boil. Add potatoes and cook until very tender when pierced, about 20 minutes. Drain and return to pan.
Add butter and 3/4 cup milk and beat with an electric mixer on medium speed, or mash with a potato masher, until smooth. Beat or stir in roasted corn and chipotles to taste. If potatoes are stiffer than you like, beat in a little more milk. Add salt and pepper to taste. Spoon into a serving bowl.