Preheat the oven to 350 degrees and line two baking sheets with parchment paper.

In a large bowl, whisk together the flour, salt, baking powder, and baking soda. Set aside.

In another large bowl, whisk together the cocoa powder and hot brewed coffee until completely combined and dissolved.

In a medium bowl, stir together the brown sugar and oil. Add this to the cocoa mixture and whisk until combined. Add the egg, vanilla, and buttermilk. Whisk until smooth.

Use a rubber spatula to gently fold the dry ingredients into the wet. Make sure to scrape down the sides and bottom of the bowl as you fold.

Use a cookie scoop to drop spoonfuls of dough onto the prepared baking sheets about 1 inch apart. (You can make these any size you like - I made about 30 small sandwiches. Bake time is for tablespoon sized dough balls.) Bake for 10 to 15 minutes, until the cookies are just starting to crack on top and a toothpick inserted in the center comes out clean. Let the cookies cool completely on the pan while you make the filling.

In a medium bowl, whisk the egg whites and sugar together. 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, stirring often.

Transfer the egg mixture to the bowl of an electric mixture fitted with the whisk attachment and beat on medium-high speed. Beat 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 until smooth and fluffy, about 5 minutes more. The buttercream will look as though it is breaking, but it will come together.

Add the salt and vanilla and beat for 5 seconds to combine.

To assemble the pies, turn half of the cookies upside down. Use a spoon to drop a large dollop of filling onto the flat side of the cookie or pipe it on. (I piped it, much easier!) Place another cookie, flat side down, on top of the filling and lightly press the cookies together. Repeat until all cookies are filled. Put the whoopie pies in the refrigerator for at least 30 minutes to set. Bring the pies to room temperature before serving.

Store tightly covered in the refrigerator.