Whoopie Pies

I took today off from my construction job *lol* to catch up on some cleaning and baking. Ever since my nephew's birthday party Abby has been pestering for whoopie pies with Oreo cookie filling. Just like the filling in Chevy's birthday cake. I whipped them up this afternoon now I'm off to finish a delicious dinner. Stay tuned.
Ingredients
BATTER
1 cup shortening
2 cups white sugar
2 eggs
1 cup sour milk*
2 tsp vanilla
4-1/2 cups all purpose flour
2 tsp salt
1 cup baking cocoa
1 cup hot water
2 tsp baking soda
FILLING
2 egg whites
4 tbsp flour
1 tbsp vanilla
4 tbsp milk
4 cups 10x sugar, divided
1 cup shortening
Directions
Preheat oven to 325 ⁰F. In a large bowl, cream together 1 cup of shortening and white sugar until light and fluffy. Beat in the eggs one at a time then stir in the vanilla and sour milk*. Combine the flour, cocoa and salt, stir into the creamed mixture. Stir together the baking soda and hot water then mix them into the batter last. Drop by tablespoonfuls onto ungreased cookie sheet. Bake for 8 minutes in the preheated oven, until firm but not over baked. Allow cookies to cool on baking sheets for a few minutes before removing to wire racks to cool completely.
To make the filling: In a medium bowl, beat together egg whites, 2 cups 10x sugar, flour, vanilla and milk. Beat in 1 cup shortening and 2 cups 10x sugar until light and fluffy. Sandwich at least 1 tablespoon of filling between the flat sides of two cookies
For the Oreo cookie filling add 2 cups ground up Oreo cookies.
*for sour milk start with 2 tsp vinegar and add enough milk to equal 1 cup