Preheat oven to 350 degrees F (175 degrees C). Line a 12-cup muffin pan with paper liners.
In a medium bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Combine the cocoa, flour, baking soda, baking powder and salt; stir into the creamed mixture alternately with the buttermilk.
Fill each muffin cup 2/3 full. Bake for 18 to 20 minutes in the preheated oven, or until a toothpick inserted into the center of a cupcake comes out clean.