Preheat oven 350°F. Prepare muffin pans with baking cups.
In medium bowl, combine flour, 2/3 cup cocoa powder, baking soda and salt.
In large bowl, beat sugar and oil with electric mixer until combined. Add eggs, one at a time, mixing well after each addition. Add vanilla; mix well. Add flour mixture alternately with sour cream, mixing until just combined. Divide batter among prepared pans, filling about 1/2 full.
Bake 16-18 minutes or until toothpick inserted comes out clean. Cool cupcakes in pans 5 minutes. Remove to cooling grid; cool completely.
For icing, in large bowl, beat shortening and butter with electric mixer until light and fluffy. Gradually add sugar and 3/4 cup cocoa, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. Icing will appear dry. Slowly add milk; beat at medium speed until light and fluffy. Pipe or spread icing onto cupcakes.