Preheat oven to 350°F. Line muffin pan with baking cups.
In medium bowl, whisk together flour, cocoa powder, baking powder, baking soda and salt; set aside.
In large mixing bowl, beat sugar and vegetable oil with electric mixer on medium speed until combined. Add eggs, one at a time, and vanilla; mix well. Add flour mixture alternately with milk, beating well after each addition. Divide batter evenly between baking cups.
Bake 18-21 minutes, or until toothpick inserted in center comes out clean. Cool cupcakes in pan on cooling grid 5 minutes. Remove from pan; cool completely.
For frosting, in large mixing bowl, beat shortening and butter with electric mixer using the paddle attachment until light and fluffy for about 1 minute. Beat in mint extract.
Gradually add sugar, starting by mixing on low speed then beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been added, frosting will appear dry. Add milk; beat at medium speed until light and fluffy. Tint icing to desired shade of green.
Frost cupcakes with mint buttercream and garnish with mini chocolate chips.