How to Make It
Submerge corn husks in a large pot filled with warm water. Soak 30 minutes; drain and discard water.
Combine stock and ancho chiles in a medium saucepan; bring to a boil. Reduce heat to medium; simmer 5 minutes. Remove from heat; let stand 5 minutes. Drain chiles over a bowl; reserve soaking liquid.
Heat oil in a large skillet over medium-high heat. Add onion; cook 2 minutes, stirring occasionally. Reduce heat to medium; stir in oregano, 1 teaspoon salt, and garlic. Sauté 7 minutes. Add chipotle; cook 1 minute. Stir in 2 cups reserved soaking liquid, tomato paste, cumin, and tomato sauce; cook 4 minutes or until mixture thickens slightly.
Place ancho chiles and onion mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until smooth. Return mixture to skillet over medium heat. Stir in chocolate; cook 1 minute. Remove 1 1/2 cups sauce from skillet; reserve. Add chicken to skillet. Remove from heat; cool to room temperature.
Combine masa harina, baking powder, and remaining 1 teaspoon kosher salt in a bowl. Cut in lard with a pastry blender or 2 knives until mixture resembles coarse meal. Add remaining 2 cups soaking liquid and 1 1/2 cups water; stir until a soft dough forms. Knead dough in bowl until smooth and pliable.
Tear a few corn husks lengthwise into 24 thin strips. Place 1 corn husk on a work surface, with narrow end facing you. Place about 3 heaping tablespoons masa mixture in center of husk, about 1 inch from top; press into a 4 x 3 1/2-inch rectangle. Spoon 2 1/2 tablespoons chicken mixture down center of masa rectangle. Fold sides of husk over filling; fold narrow end up to form a packet. Tie strip around tamale to secure the packet. Repeat procedure with remaining husks, dough, filling, and husk strips.
Poke holes in bottom of a 13 1/2 x 9 1/2-inch shallow disposable aluminum pan; cut corners from rim down to bottom to make it pliable. Fit pan into bottom of a 6-quart oval or rectangular slow cooker; pour remaining 1 cup water into pan. Line bottom of pan with a damp kitchen towel. Stand tamales in prepared pan, with open ends facing upright. Cover slow cooker (weighing down lid if necessary to keep it closed); cook on HIGH for 4 hours or until masa is set. Let tamales stand 10 minutes. Serve with remaining sauce.