Preheat the oven to 400ºF. Line a baking sheet with parchment paper.
In the bowl of a stand mixer, cream together the sugar, salt, butter, vanilla and egg. Mix well, scraping the sides of the bowl as needed. 1 cup granulated sugar, 1/4 teaspoon salt, 1/2 cup salted butter, 1 teaspoon vanilla extract, 1 large egg
In a separate bowl, sift together the flour, baking soda and cocoa. Add this to the creamed butter mixture alternately with the coffee and buttermilk. 2 cups all-purpose flour, 1 teaspoon baking soda, 1/2 cup unsweetened cocoa powder, 1/2 cup strongly brewed coffee, 1/2 cup buttermilk
Drop the batter by teaspoonfuls onto the prepared pan. Bake for 7-8 minutes. Set the cakes aside to cool.
In a stand mixer bowl, whip the room temperature butter on medium speed until it is almost white in appearance. 3/4 cup salted butter
Add the powdered sugar and cocoa powder. Mix on low speed. 2 1/2 cups powdered sugar, 2 tablespoons cocoa powder
Add in the cooled coffee and whip the buttercream on medium speed for 3-4 minutes, or until light and fluffy. 2 tablespoons strongly brewed coffee
Choose two whoopie pies that are about the same size. Spread the buttercream between the two cakes.
This should make about 18 whoopie pies that are 2 1/2" in diameter. Store in an airtight container.
Notes
If the whoopie pie batter seems to thin, it may be because of the fat content of your milk. I used 1% milk, and didn't need to add any flour. You can add a bit of flour to thicken the batter, if needed, a teaspoon full at a time.
To make the strongly brewed coffee, I used 3 tablespoons of ground espresso and brewed it with 3/4 cup water. You can also use espresso powder, which is instant espresso that easily dissolves in hot water.
Refer to the article above for more tips and tricks.The calories shown are based on the recipe making 18 pies, with 1 serving being 1 whoopie pie. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information. **