Preheat the oven to 350 degrees. Line 2 baking sheets with parchment paper (or grease them with oil).

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 1/2 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. Fill a piping bag with batter and pipe quarter-sized dollops 1-inch apart onto greased baking sheets. Bake for 10 minutes, until a toothpick inserted into the center of one whoopie comes out clean. Place cookies onto wax paper to cool while you bake the remaining batches.

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 milky white, about 2-3 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 butter cream looks like it is breaking, don't worry, it will eventually come together.

Add the salt and Irish Cream and beat for 5 seconds to combine.

Pipe filling onto the flat sides of half of the whoopies and top with remaining half.