1. Preheat oven to 350°. Beat butter at medium speed with an electric mixer until fluffy; gradually add sugar, beating well. Add eggs, 1 at a time, beating just until blended.
2. Combine flour, cocoa, and salt. Stir together sour cream and baking soda. Add flour mixture to butter mixture alternately with sour cream mixture, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Stir in half of chopped chocolate.
3. Place paper baking cups in a 12-cup muffin pan, and coat with cooking spray; spoon batter into cups, filling two-thirds full.
4. Bake at 350° for 18 to 20 minutes or until a wooden pick inserted in center comes out clean. Remove from pan to a wire rack, and cool completely (about 30 minutes).
5. Pipe Marshmallow Frosting onto cupcakes; sprinkle with pecans and remaining chopped chocolate.