1 1/2 cups (3 sticks) unsalted butter, plus more for pans
3/4 cup Cacao Barry Extra Brute (or other premium dark) cocoa powder,
1/2 cup boiling water
2 1/4 cups sugar
1 tablespoon pure vanilla extract
4 large eggs, lightly beaten
2 1/2 cups all-purpose flour
1 teaspoon baking soda
1/2 teaspoon salt
1 cup buttermilk
Preheat oven to 350 F. Butter three 8″ x 2″ round cake pans, line with parchment rounds, and dust with flour, tapping out the excess. Add boiling water to sifted cocoa powder in medium bowl and whisk; set aside to cool.
In a stand mixer fitted with the paddle attachment, beat the butter, sugar, and vanilla on medium-high speed until lighter in color and slightly increased in volume, 3 to 5 minutes. Lower the speed to medium and add the eggs, one at a time, mixing until each is fully incorporated before adding the next.
Sift the flour and baking soda into a medium bowl. Add the salt to the dry ingredients after sifting and whisk together. Whisk buttermilk into cocoa mixture. Alternate dry ingredients and buttermilk into butter mixture, beginning and ending with dry ingredients. Mix until just incorporated, or finish by hand gently.
Divide the batter evenly between the prepared pans. If possible, weigh the batter in each cake pan to ensure even layers. Smooth with small offset palette knife, and bake for about 35 minutes, rotating once after 20 minutes. Cake is done when toothpick or skewer comes barely clean. Try not to over-bake. I tend to under-bake a few moments, so the skewer is a little bit gummy. This works well for a moist chocolate cake (not vanilla). Let pans cool on wire rack for 20 minutes, then invert cakes onto racks, gently, peeling away parchment rounds. Let cool completely.