Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a mixing bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt.
In another bowl, beat the butter and sugar until light and fluffy. Add the egg and vanilla extract, and mix well.
Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Mix until just combined.
Drop spoonfuls of batter onto the prepared baking sheet, spacing them about 2 inches apart.
Bake for 10-12 minutes, or until the cookies are set. Let them cool on the baking sheet for a few minutes before transferring to a cooling rack.
Once cooled, spread your favorite filling on the flat side of one cookie and top with another cookie to make a sandwich.