Preheat oven to 350°F (175°C). Line a muffin tin with cupcake liners.
In a large bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.
In a separate bowl, cream together sugar and butter until light and fluffy. Beat in eggs one at a time, then stir in vanilla extract.
Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined.
Fold in the crushed Oreos.
Fill cupcake 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
For extra chocolate flavor, use dark cocoa powder. You can also add chocolate chips to the batter.