- Preheat oven to 350°. Line 3 cookie sheets with parchment paper.
- In a large bowl, whisk together the flour, salt, baking powder, and baking soda.
- In a separate large bowl, stir together the brown sugar and oil until combined.
- In a measuring cup or small bowl, whisk together the cocoa powder and hot coffee until the powder is completely dissolved. Add this to the brown sugar mixture and whisk to combine. Add the egg, vanilla and buttermilk to the mixture and whisk until smooth.
- Using a rubber spatula, fold the dry ingredients into the wet ingredients until combined.
- 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. You can also use a tablespoon and your fingers.
- Bake for 5 minutes, rotate the trays, and bake until the cookies bounce back in the center when touched, or a toothpick comes out clean, about 3 more minutes. Let the cookies cool completely while you make the filling.
- In the bowl of your stand mixer or other large bowl, whisk the egg whites and sugar together.
- Rest the bowl over a pot of simmering water but do not let the water touch the bottom of the bowl. Heat the mixture while whisking, until the sugar is completely dissolved and the color is milky white, about 2-3 minutes or to 140 degrees.
- Beat the egg mixture with the whisk attachment on medium-high speed until smooth and fluffy, about 5 minutes.
- Remove the whisk attachment and replace with the paddle attachment. Add the butter in 3 or 4 additions, beating on medium-high speed as you go 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, vanilla, and melted chocolate and beat briefly to combine.
Assemble the Whoopie Pies:
- Use an ice cream scoop, tablespoon, or piping bag to drop a large dollop of filling onto the flat side of half of the cookies.
- 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. Extra filling can be kept in the fridge for up to a week. Put the whoopie pies in the refrigerator for about 30 minutes to firm up before serving.