Preheat the oven to 400 degrees F. Lightly grease a 10-inch cast-iron skillet with butter.
Toss the sweet potatoes with the melted butter, cocoa powder, sugar, cinnamon, chipotle powder and salt in a large bowl until well coated. Make a single concentric layer of overlapping sweet potato slices in the bottom of the skillet, then generously sprinkle with 1/2 cup graham cracker crumbs. Repeat 2 more times.
Evenly pour the half-and-half over the sweet potatoes, then cover the pan with aluminum foil; bake for 30 minutes. Remove the foil and cover the sweet potatoes with the miniature marshmallows. Turn the oven to broil and place the pan at least 6 inches away from the flame. Toast until the marshmallows are brown, 3 to 5 minutes. Cool for 10 minutes before serving.