In a separate bowl, whisk together the buttermilk, sour cream, and coffee. 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.