Preheat oven to 350°F (175°C). Line a muffin tin with paper liners.
In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.
In a large bowl, cream together butter and sugar until light and fluffy. Beat in eggs one at a time, then stir in vanilla.
Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined.
Stir in the crushed Oreos.
Fill the muffin liners about 2/3 full.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Let cool in the tin for a few minutes before transferring to a wire rack to cool completely.
Notes
These cupcakes are best served the same day they are baked. Store leftovers in an airtight container at room temperature.