Whoopie Pies food styling and photos by kake © Sarah Phillips

I inherited this recipe for Whoopie Pies from Tami Smith, which I slightly modified and added a filling recipe to. This recipe has been passed from her mother's side of the family (Pennsylvania Dutch descendants) to her. It's as authentic as you can get.
COOKIE RECIPE HELP 

Sarah Phillips writes in the NY Times blog post: http://www.nytimes.com/info/cake/
Aug. 2, 2010 12:12 PM ET Gaining in popularity once again, the whoopie pie is not a pie at all, but is thought to have evolved in Depression-era New England and Pennsylvania Amish country from left-over cake batter, traditionally chocolate. Dropped onto a cookie sheet and spread into round circles a few inches in diameter, the batter bakes into a mound shape, with a flat side. Two mounds are matched and sandwiched together with a fluffy white frosting or marshmallow fluff in between.
—- Sarah Phillips, founder, CraftyBaking.com, a baking advice and recipe site

INGREDIENTS
Cookies:
3/4 cup boiling water
3/4 cup cocoa powder, Dutch-process*

4 cups unbleached all-purpose flour; spoon into measuring cup and level to rim
2 1/2 teaspoons baking powder
1/2 teaspoon salt

2 cups sugar
1/2 cup crisco shortening or unsalted butter (1 stick or 8 tablespoons)
1 large egg
2 large egg yolks

2/3 cup buttermilk

*NOTE:
If you use:
3/4 cup Natural cocoa powder, instead of 3/4 cup Dutch process, substitute the 2 1/2 teaspoons baking powder with
1 1/2 teaspoons baking powder plus 1/4 teaspoon baking soda

Filling:
1 recipe Tami’s or Perfect Buttercream or the Homemade Marshmallow Creme (Cream) 

INSTRUCTIONS
1. Position and oven shelf in the middle of the oven and preheat the oven to 350 degrees F. Lightly grease baking or line sheets with parchment paper or silpat mats.

2. In a bowl or measuring cup, mix the boiling water and cocoa, stir until smooth, set aside to cool.

3. Combine and sift the flour, baking powder and salt. Set aside.

4. In a mixing bowl of a mixer fitted with a paddle attachment, cream the Crisco and sugar. With the mixer on low, add the egg and egg yolks, and beat. Scrape the bowl.

5. With the mixer on low, slowly add in 1/3 of the flour mixture at the side of the bowl, and then, 1/2 of the buttermilk, immediately followed by 1/2 of the cocoa powder mixture.
Stop the mixer and scrape the side of the bowl.

Repeat, the sequence again, starting with 1/3 of the flour mixture, 1/2 of the buttermilk, 1/2 of the cocoa powder mixture and then, ending with the last 1/3 of the flour mixture.
NOTE: This batter will cake-like in appearance.

Stop the mixer and scrape the side of the bowl.

6. Drop batter by the 1/4 cup onto prepared baking sheets, making sure you make 18 individual cakes. With the back of a spoon spread batter into 4-inch circles, leaving approximately 2 inches between each cake. Make sure the cookie batter domes slightly in the middle and the sizes are consistent.

7. Bake 10 to 12 minutes or until the center is puffed and the cookies spring back (and feel spongy) when lightly pressed. Remove from the cookie sheets and cool a wire rack.

FILL
1. When cool, match up equal sized cookies, spread the flat side (bottom) of one chocolate cake with a generous amount of filling by using a piping bag or spoon.

2. Top with the flat side of another cookie, gently pressing down to distribute the filling.

STORAGE
The unfilled Whoopie Pie halves can be stored at room temperature, well wrapped, for 2 to 3 days, the same as any cake recipe. They can be frozen for a month or more. Thaw at room temperature in their wrappers. Do not refrigerate.

The filled Whoopie Pies can be stored at room temperature the same way, as above, when filled with Tami's or Perfect Buttercream. Filled pies can be frozen for a month or more.

However, if filled with the Homemade Marshmallow Creme (Cream), the filling is best used the day it is made. However, freezing filled pies may alter the filling's texture.