Cream together the butter and sugar in a large mixing bowl until light and fluffy. Add the eggs, one at a time, beating for 30 seconds between additions. Sift the flour, baking powder, baking soda, and salt. In a seperate bowl, combine the buttermilk and vanilla. Add alternating increments of the flour mixture and the buttermilk mixture to the batter until blended.
Pour into prepared muffin cups and bake for 15 minutes, or until a toothpick inserted in the middle comes out clean. Let the cupcakes rest for 2 minutes before transferring them to a wire rack to cool.
Mix together the powdered sugar, buttermilk, and cocoa for the choclate glaze. Spread over cooled cupcakes.