3½ cups all-purpose flour
½ teaspoon salt
1¼ teaspoons baking powder
1 teaspoon baking soda
¾ cup dark unsweetened cocoa powder (like Valrhona)
2 teaspoons instant espresso powder
½ cup hot coffee
2 cups firmly packed light brown sugar
¾ cup canola oil
1 large egg
1 teaspoon pure vanilla extract
½ cup buttermilk
Cream Cheese Filling
3 cups confectioners' sugar
½ cup (1 stick) unsalted butter, softened
8 ounces cream cheese, softened
1 teaspoon pure vanilla extractRed and yellow food coloring
Make the Cookie-Cake
Preheat the oven to 350ºF. Line two baking sheets with parchment paper or Silpat.
In a large bowl, whisk together the flour, salt, baking powder, and baking soda, and set aside.
In another large bowl, whisk together the cocoa powder and espresso powder. Add the hot coffee and ½ cup hot milk and whisk until both powders are completely dissolved.
In a medium bowl, stir the brown sugar and oil together. Add this to the cocoa mixture and whisk until combined. Add the egg, vanilla, and buttermilk and whisk until sooth.
Use a rubber spatula to gently fold the dry ingredients into the wet ingredients. Make sure to scrape down the sides and bottom of the bowl as you fold.
Use a small ice cream scoop with a release mechanism to drop heaping tablespoons of the dough onto the prepared baking sheets about 1 inch apart. Bake for 10 to 15 minutes, until the cookies are just starting to crack on top and a toothpick inserted into the center of a cookie comes out clean. Let the cookies cool completely on the pan while you make the frosting.
Make the Filling
Sift the confectioners' sugar into a medium bowl and set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter until it is completely smooth, with now visible lumps. Add the cream cheese and beat until combined.
Add the confectioners' sugar and vanilla and beat until smooth. Add the yellow and red food coloring a few drops at a time to create the desired shade of orange. Be careful not to overbeat the filling, or it will lose structure. (The filling can be made 1 day ahead. Cover the bowl tightly and put it in the refrigerator. Let the filling soften at room temperature before using.)
Assemble the Whoopie Pies
Turn half of the cooled cookies upside down (flat side facing up).
Use a spoon to drop a large dollop of filling onto the flat side of the cookie. Place another cookie, flat side down, on top of the filling. Press down slightly so that the filling spreads to the edges of the cookie. Repeat until all the cookies are used. Put the whoopie pies in the refrigerator for about 30 minutes to firm up before serving.
The whoopie pies will keep for up to 3 days, on a parchment-lined baking sheet covered with plastic wrap, in the refrigerator. Bring the whoopies to room temperature before serving.