1. In a small bowl, mix butter, brown sugar, and cinnamon. Set sweet potato halves, cut side up, in a baking dish, and rub cut sides equally with butter mixture. Bake in a 350° oven until soft when pressed, about 1 hour.
2. Scoop flesh from sweet potatoes (discard skins) into a bowl; mash with a fork until smooth.
3. In a 1- to 2-quart pan over low heat, melt lard. Pour in chicken broth and stir until warm. Add broth mixture to sweet potatoes; mix well. Gradually add masa flour, stirring until well blended. Stir in baking powder and chipotles and season with salt and pepper to taste.
4. Tear 3 or 4 corn husks into long, thin strips. Lay a whole husk flat on a work surface, smooth side down, with one long edge closest to you. Spoon a scant 1/3 cup filling lengthwise down center of husk. Fold bottom edge over filling and roll up tamale. Tie each end with a thin strip of husk. Repeat until all filling is used.
5. In an 8- to 10-quart pan, position a rack at least 1 inch above 1 inch of water; bring to a boil over high heat. Set tamales on rack, lower heat to maintain a simmer, cover, and steam until filling is firm, about 1 hour, adding water to pan as needed to maintain 1-inch depth. Remove tamales and let stand at least 10 minutes before serving.
Nutritional analysis per tamale.
NOTES: Dried corn husks are available in some well-stocked supermarkets and in Mexican markets. Before using, separate husks and discard silks; soak husks in hot water until pliable, about 20 minutes. You will need about 15 large, well-shaped husks for the tamales and 3 to 4 to tear into strips for tying, but we suggest soaking a few extra in case some tear in the process. You can make and steam the tamales up to a day ahead; cover and chill. Reheat in steamer 15 to 20 minutes.