Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper and set them aside.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.
In a large mixing bowl, beat the butter and sugar together using an electric mixer on medium speed until light and fluffy.
Add the egg, red food coloring, and vanilla extract to the butter mixture. Beat until well combined.
Alternate adding the dry mixture and buttermilk to the wet mixture, starting and ending with the dry ingredients. Mix until just combined.
Using a cookie scoop or tablespoon, drop rounded tablespoons of batter onto the prepared baking sheets, spaced about 2 inches apart.
Bake for 10-12 minutes, or until the tops spring back when lightly pressed. Let cool on the baking sheets for a few minutes before transferring to a wire rack.
While the whoopie pies are cooling, prepare your filling. You can use a simple cream cheese frosting or any filling of your choice.
Once the cookies are completely cool, spread a generous amount of filling on the flat side of one pie and top with another pie.
