How to Make a Whoopie Pie

The whoopie pie is a soft, luscious chocolate cake which hails from Amish culture in the northern USA states around Pennsylvania. It has even been adopted as the main pie food of the state of Maine. The key is a filling of swiss meringue butter cream filling between the two cake halves. Impossible to just walk away from without trying to eat them. 

Ingredients (Whoopie Pie):

For the cake-cookies:

• 2 cups/200g all-purpose flour
• 2/3 to 3/4 cup unsweetened cocoa powder – usually Dutch processed.
• 1 teaspoon baking powder
• 1/4 to 1/2 teaspoon baking soda
• 1/2 teaspoon salt
• 1/2 to 3/4 cup unsalted butter, softened
• 1 cup/200g granulated sugar
• 2 large eggs
• (Optional) 1/4 cup strong coffee
• (Optional) 1 teaspoon vanilla extract
• 1/2 to 3/4 cup buttermilk OR 3/4 cup of milk with 1/2 cup of full-fat sour cream added. Never use low-fat or fat-free sour cream.

For the filling:

• 1/2 cup unsalted butter, softened
• 1 cup powdered sugar
• 1 teaspoon vanilla extract
• 1/4 cup heavy cream

For a Swiss meringue buttercream filling:

• 5 egg whites only in a clean bowl at ambient/room temperature
• 1 and 3/4 cups caster sugar or fine sugar
• 1 pinch of salt
• 2 cups unsalted butter at room temperature
• 1 Tbsp vanilla extract or 1/4 Baileys etc.

Preparation (Whoopie Pie):

1. Preheat your oven to 350°F (180°C). Line a baking sheet with parchment paper.
2. In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.
3. Put the flour blend through a sieve to sift it. It is ideal to have it sit on your mixing bowl. Put to one side as this is your dry ingredient!
4. In a large mixing bowl (Kitchen Aid mixer with paddle attachment is ideal for this step), beat (cream) the butter and sugar together until light and fluffy.
5. In another bowl we create the wet ingredient mixture. Beat the eggs with the buttermilk to form a smooth liquid. You could add the coffee too if you wanted to add an extra note to the chocolate flavour. 
6. To the creamed butter and sugar, gradually mix in a cup of dry ingredients, alternating with the wet ingredients (eggs/buttermilk mix), until the batter is smooth. Keep alternating until all the mixtures are added. Some chefs get decent results just adding the butter and then the wet ingredients whole into the flour mix with steady beating going on. I think the texture using the second method is just a little bit more cookie like than cake like but it’s a matter of taste in texture.
7. You can warm the baking trays with parchment etc. up for 5 minutes in the oven. It helps create the smooth tops to the cake/cookie.
8. Drop tablespoonfuls of the batter onto the prepared baking sheet on a tray. You could use a tray lined with Silpat, spacing them about 2 inches apart. Shape the balls of dough using wet fingers into roundish balls. 
9. Bake the cookies for 9 minutes at 375ºF or 10-12 minutes at 350ºF, or until they are set and spring back when lightly touched.
10. Allow the cookies to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.
11. To make the ordinary filling, beat the butter and powdered sugar together until light and fluffy.
12. Mix in the vanilla extract and heavy cream until the filling is smooth and creamy.
13. If you make the swiss meringue filling, just put the egg whites in a mixing bowl, then the sugar. Whisk well! Place over a pot of simmering water but make sure the bowl does not touch the water. Use the steam heat to bring the temperature to 160ºF. Keep whisking as it warms which is key here. Add a pinch of salt. Once it has roughly reached that temperature, pour the egg white mixture into a clean bowl. Here the Kitchen Aid mixer comes in handy with its whisk attachment.  Mix on high to form a creamy meringue like structure. It should cool a bit too.
14. With the mix on low, add cool butter, tablespoon at a time.  The meringue starts to deflate because the fat is destabilizing the structure. If it is too liquid like because the meringue was too warm, just pop into the fridge to cool down further and then start whisking again. Add the vanilla extract or which ever flavour you like. The mixture should suddenly thicken to look like very thick clotted cream. 
15. To assemble the whoopie pies, spread a dollop of filling on the flat side of one cookie, and then sandwich it with another cookie. Or you can put the filling into a piping bag with a no. 1 attachment. Gently press together. 
16. Repeat with the remaining cookies and filling.

Your whoopie pies are now ready to enjoy! Store any leftovers in an airtight container at room temperature for up to 3 days.