In a large mixing bowl, whisk together the sugar, flour, cocoa powder, baking powder, baking soda, and salt. In a separate bowl, combine the eggs, buttermilk, oil, and vanilla extract, mixing until well blended. Pour the wet mixture into the dry ingredients and stir until fully combined. Slowly add the boiling water, mixing carefully—the batter will be quite thin. Divide the batter evenly between the prepared pans.