Preheat oven to 375 degrees F.
Lay chiles on their sides on rack of a gas burner and turn flame on high. (Or put chiles on rack of broiler pan about 2 inches from heat.) Roast chiles, turning with tongs, until skins are blackened, 5 to 8 minutes. Transfer chiles to a bowl and let stand, covered, 15 minutes.
Rub skins off chiles with paper towels and discard stems and seeds (if desired, devein to further reduce heat). Cut the chiles into 1/4-inch-thick strips.
Bring cream with garlic just to a simmer and remove from heat. Peel potatoes and cut crosswise into 1/16-inch-thick slices using mandoline.
Arrange one-third of potatoes evenly in bottom of a well-buttered 1 1/2-quart shallow baking dish, overlapping them slightly, and sprinkle with salt, to taste, and 1/2 of chiles. Repeat, ending with a layer of potatoes. Remove garlic from cream and pour cream over potatoes. Cover dish with foil and bake in lower third of oven 45 minutes. Remove foil and bake, basting the top occasionally, until gratin is golden brown on top and bubbling, about 30 minutes more. Cool slightly before serving.
Cooks' note: Gratin can be baked 1 day ahead, then cooled completely before being chilled, covered. Bring to room temperature before reheating, covered, in a preheated 350 degree F oven.
Recipe courtesy of Gourmet Magazine