Preheat the oven to 350 degrees F. Place liners in cupcake pan.
Cream the butter and 2 sugars on high speed until light and fluffy, approximately 5 minutes. (In the bowl of an electric mixer fitted with a paddle attachment is ideal, but a hand mixer can be used as well).
Lower the speed to medium, add the eggs and the vanilla and mix well.
In a separate bowl, whisk together the buttermilk and sour cream. In another bowl, sift together the flour, cocoa, baking soda, and salt. On low speed, add the buttermilk mixture and the flour mixture alternately in thirds to the mixer bowl, beginning with the buttermilk mixture and ending with the flour mixture. Mix only until blended. Fold the batter with a rubber spatula to be sure it's completely blended.
Divide the batter among the cupcake pans (about ¾ of the way for each cupcake). Bake in the middle of the oven for 20 to 25 minutes, until a toothpick comes out clean. Cool for 10 minutes, remove from the pans, and allow to cool completely before frosting.