3 1/2 cups all-purpose flour
1/4 teaspoon salt
1 1/4 teaspoons baking powder
1 1/4 teaspoons baking soda
3/4 cup dark unsweetened cocoa
2 teaspoons instant espresso powder
1/3 cup hot coffee
2 cups firmly packed light brown sugar
3/4 cup canola oil
1 large egg
1 teaspoon vanilla
1/2 cup buttermilk, shaken
5 large egg whites, for filling
1 1/2 cups sugar, fo filling
2 cups (4 sticks) unsalted butter, cut into 1/2-inch cubes, cool but not cold, for filling
1/4 teaspoon salt, for filling
1 teaspoon pure vanilla extract, for filling
To Make the Chocolate Cookies: Preheat the oven to 350 degrees F. Line two baking sheets with parchment paper.
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 water 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 smooth.
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 centre of a cookie comes out clean.
Let the cookies cool completely on the pan while you make the Swiss vanilla filling.
To Make the Swiss Vanilla Filling: In a medium bowl, whisk the egg whites and sugar together (remember to substitute the sugar for the optional peanut butter filling).
Set the bowl over a pan of simmering water but do not let the water touch the bottom of the bowl. Heat the mixture until the sugar is completely dissolved and the color is a milky white, about 2 to 3 minutes.
Transfer the egg mixture to the bowl of an electric mixer fitted with the whisk attachment and beat on medium-high speed (start slowly at first) until smooth and fluffy, about 5 minutes. Remove the whisk attachment and replace with the paddle attachment.
Add the cubed butter and beat on medium-high speed (start slowly at first) until smooth and fluffy, about 5 minutes. If the buttercream looks like it is breaking, don't worry, it will eventually come together.
Add the salt and vanilla and beat for 5 seconds to combine.
Variation: Replace the sugar with 1 cup granulated sugar and 1/2 cup light brown sugar (packed tightly). Fold in ¼ cup unsalted smooth peanut butter after adding the vanilla extract.
To Assemble the Whoopie Pies: Turn half of the cooled cookies upside down (flat side facing up).
Use an ice cream scoop or a tablespoon 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.