Preheat oven to 350°. In a large bowl, beat sugar, oil, pumpkin and eggs until well blended. Combine flour, baking soda, salt, baking powder and cinnamon; gradually beat into pumpkin mixture until well blended.
Fill paper-lined muffin cups two-thirds full. Bake 18-22 minutes or until a toothpick inserted in center comes out clean. Cool 10 minutes before removing from pans to wire racks to cool completely.
For filling, combine cornstarch and milk in a small saucepan until smooth. Bring to a boil, stirring constantly. Remove from heat; cool to room temperature.
In a large bowl, cream shortening, butter and confectioners' sugar until light and fluffy. Beat in vanilla if desired. Gradually add cornstarch mixture, beating until smooth.
Using a sharp knife, cut a 1-in. circle 1 in. deep in the top of each cupcake. Carefully remove tops and set aside. Spoon or pipe filling into cupcakes. Replace tops. If desired, add a clove pumpkin stem to the tops. Yield: about 1-1/2 dozen.