Preheat the oven to 300°F.
Lightly butter three 9 inch round cake pans, line the bottoms with parchment paper, and butter the paper. Use unsweetened cocoa powder to "flour" the pans and set aside.
butter, 1½ cups (144 grams) unsweetened cocoa powder
Sift the flour, cocoa, baking powder, baking soda, and salt into a medium bowl.
2½ cups (312 grams) bleached, all-purpose flour, 1½ cups (144 grams) unsweetened cocoa powder, 2 teaspoons baking powder, ¾ teaspoon baking soda, 1 teaspoon (3 grams) kosher salt
In a bowl of a stand mixer or large bowl if using a hand mixer, beat the eggs on medium speed until pale yellow, about 3 to 5 minutes.
3 eggs
Add the sugar and beat for a couple of minutes.
3 cups (600 grams) granulated sugar
Add the cooled chocolate and mix, then add coffee, buttermilk, oil, and vanilla.
3 ounces (85 grams) semisweet or bittersweet chocolate, 1½ cup buttermilk, ¾ cup neutral vegetable oil, 1 teaspoon vanilla, 1½ cup coffee, cooled
Add the dry ingredients and combine on low speed for 1 minute or by hand until just combined. Your chocolate cake batter may not be perfectly smooth, but don't leave any pockets of the cocoa mixture.
Divide the three chocolate cake layers equally into the prepared pans. Before putting them in the oven, knock them gingerly on the countertop once or twice to remove any air bubbles.
Bake for about 60 minutes, until a toothpick inserted into the center comes out clean and the edges are just starting to pull away from the side of the pan.
Cool cake layers in the pan. You may need to run a spatula or knife around the edge of the cake pans to get the layers to release from the pan.