In a large mixing bowl, combine the sugar, oil, pumpkin and eggs. In another bowl, whisk the flour, baking soda, salt, baking powder and cinnamon together; add to pumpkin mixture and beat until well mixed.
Fill paper-lined muffin cups two-thirds full. Bake at 350 degrees for 18-22 minutes or until a toothpick comes out clean. Cool for 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 the heat; cool to room temperature.
In a large mixing bowl, cream shortening, butter and confectioners' sugar. Beat in vanilla if desired. Gradually add the cornstarch mixture, beating until light and fluffy.
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. Yield: about 1-3/4 dozen (I got 2 dozen with no problem)