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.
The mocha whoopie pies can be made a day in advance. The flavors blend, the buttercream softens the cakes, making them extra delicious the second day. Store leftovers in an airtight container for 2-3 days.They should also freeze really well! Wrap them individually in plastic wrap, then place them in a zippered bag or other other container.
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. **