For the Cake:
1 cup unsalted butter
1/3 cup Dutch-process cocoa powder
1 cup hot water
2 cups all-purpose flour
1 teaspoon espresso powder
2 cups granulated sugar
1 teaspoon baking soda
½ teaspoon salt
½ cup buttermilk
1 teaspoon vanilla extract
For the Filling:
1/3 cup Dutch-process cocoa powder, sifted
½ cup semisweet chocolate chips
¼ teaspoon salt
½ cup heavy cream
1 teaspoon vanilla extract
¼ cup unsalted butter, at room temperature
½ cup vegetable shortening, at room temperature
4 cups powdered sugar, sifted
For the Icing:
1 cup semisweet chocolate chips
1 tablespoon light corn syrup
½ teaspoon vanilla extract
1/3 cup heavy cream
¾ cup powdered sugar, sifted
1. Preheat oven to 350 degrees F. Line a 9x13-inch pan with parchment, then grease and flour the pan; set aside.
2. Make the Cake: In a small bowl, melt the butter, then stir in the cocoa powder and hot water until the cocoa is completely dissolved; set aside.
3. In a large bowl, whisk together the flour, espresso powder, sugar, baking soda and salt; set aside. Pour the cocoa mixture over the dry ingredients, stirring to blend completely. Add the buttermilk, eggs and vanilla, and stir with a rubber spatula until completely combined.
4. Pour the batter into the prepared pan. Bake for 30 minutes, or until a toothpick inserted in the center comes out clean and the cake begins to pull away from the edges of the pan. Remove from the oven and cool completely on a wire cooling rack.
5. Make the Filling: Place the cocoa powder, chocolate chips, salt and heavy cream in a microwave-safe bowl. Microwave until the cream is very hot, and the chips have softened, about 1 minutes. Remove from the microwave and stir until the chips are melted and the mixture is smooth, microwaving again briefly if necessary. Stir in the vanilla and set aside.
6. In a large bowl, beat together the butter, shortening and half of the powdered sugar on medium speed until fluffy, about 3 to 5 minutes. Add the chocolate mixture and continue beating until smooth. Scrap the sides of the bowl and add the remaining 2 cups powdered sugar. Beat again until the filling is fluffy.
7. Assemble the Cakes: Turn the cooled cake out of the pan and cut it in half crosswise, to make two 9"x6.5" rectangles. using a long serrated knife, slice through the middle of each rectangle to make two halves (a top and a bottom).
8. Spread half of the filling on one bottom piece to the edge of the cake. Repeat with the other bottom piece. Top the bottom pieces with the top pieces. You should now have two chocolate-filled 9"x6.5" cakes.
9. Make the Icing: Combine the chocolate chips, corn syrup vanilla and heavy cream in a microwave-safe bowl. Microwave until the cream is very hot, and the chips have softened, about 1 minute. Stir till the chips have melted and the mixture is smooth, microwaving again briefly if necessary. Add the powdered sugar and beat on medium speed until smooth.
10. Spread the icing evenly over the two cakes. Working with one cake at a time, slice into nine 2x3-inch snack cakes. Repeat with the remaining cake for 18 total snack cakes.
Store the cakes at room temperature in an airtight container (you can wrap them individually in plastic wrap to keep them extra-fresh) for up to 3 days.