In a large mixing bowl, combine the sugar, oil, pumpkin and eggs.
Combine the flour, baking soda, salt, baking powder and cinnamon; add to pumpkin mixture and beat until well mixed.
Fill paper-lined muffin cups two-thirds full.
Bake at 350° 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.
Note: for the filling: suggested by a fellow foodie: When making pumpkin whoopie pies, use cream cheese instead of shortening and add a pinch of nutmeg in the filling. The taste is exceptional with with the pumpkin.