Pre-heat oven to 350°F.
Lightly butter two 9-inch round cake pans. Line the bottom of each pan with a round of parchment paper. Dust the pans with flour, tapping out the excess.
In a small heatproof bowl, whisk together the boiling water and cocoa until smooth. Let cool completely.
In a medium bowl, sift together the flour, baking soda, and salt.
In a large bowl, beat on medium-high speed (use a hand held or stand-up mixer) the sugar and butter until the mixture is light in color and texture, about 3 minutes.
Beat in the eggs, one at a time.
Beat in the vanilla and the cooled cocoa mixture.
Reduce the speed to low and add the flour mixture in 3 additions alternately with the buttermilk in 2 additions, beginning and ending with the flour mixture and stopping to scrape down the bowl as needed, beating until smooth.
Divide the batter evenly between the prepared pan and smooth the tops.
Bake the cakes until they begin to pull away from the sides of the pans, 35 - 40 minutes.
Transfer to wire racks and let cool in the pans for 15 minutes.
Run a knife around the inside of each pan to release the cake and invert the pans onto the racks, lift off the pans, and peel off the parchment paper.
Turn each cake right side up and let cool completely.