Preheat oven to 375°F (190°C). Line baking sheets with parchment paper.
In a large bowl, cream together the butter and sugar until light and fluffy.
Beat in the eggs one at a time, then stir in the vanilla.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined.
Drop by rounded tablespoons onto the prepared baking sheets.
Bake for 8-10 minutes, or until a toothpick inserted into the center comes out with moist crumbs.
Let cool on the baking sheets for a few minutes before transferring to a wire rack to cool completely.
Notes
These whoopie pies are best served the day they are made, but can be stored in an airtight container at room temperature for up to 3 days.