Prepare Batter: Preheat the oven to 350°F. Sift the flour, cocoa powder, baking soda, and salt together in a medium bowl and set aside.
Cream the butter and sugar together until light and fluffy. Add the egg and vanilla and mix well. In a separate bowl, dissolve the instant espresso powder in the buttermilk and mix well.
Gradually add the dry ingredients to the butter mixture, alternating with the espresso mixture, starting and ending with the dry ingredients.
Bake the Cookies: Line baking sheets with parchment paper.
Drop tablespoon-sized scoops of batter onto the prepared baking sheets, spacing them at least 2 inches apart. Bake for 10-12 minutes or until the edges are set and the cookies spring back when lightly touched.
Allow to cool completely on wire racks.
Prepare Filling: For the filling, beat 1/2 cup of unsalted butter, 1 cup of confectioners’ sugar, and 2 tablespoons of heavy cream together in a medium bowl until smooth. Add 1 teaspoon of vanilla extract and 1 tablespoon of instant espresso powder and mix well.
Assemble the Whoopie Pies: Pair up the cookies, and using a piping bag and a star tip, pipe the espresso filling onto the flat side of one cookie. Sandwich with the other cookie, flat side down, and press together lightly. Repeat with the remaining cookies and filling.