Preheat your oven to 350°F / 177°C. Grease two 9-inch round cake pans and line the bottoms with parchment paper. Sift the all-purpose flour, granulated sugar, dark cocoa powder, baking powder, baking soda, and salt together into a large bowl to aerate the starches.
Add the buttermilk, vegetable oil, room temperature eggs, and vanilla extract to the dry mixture. Whisk vigorously for two minutes until the batter becomes completely smooth, pale, and fully emulsified.
Carefully pour the boiling water or hot espresso into the batter. Whisk gently until fully incorporated. The batter will be extremely thin and watery, which is necessary to melt the cocoa fats and hydrate the flour proteins perfectly.
Divide the liquid batter evenly between your prepared cake pans. Bake for 30 to 35 minutes. Test the center with a digital thermometer; it is perfectly baked when the internal temperature reads between 205°F / 96°C and 210°F / 99°C.
Remove the pans from the oven and let them rest on a wire cooling rack for exactly 10 minutes. Run a thin knife around the edges, invert the cakes onto the racks, remove the parchment paper, and let them cool completely to room temperature before frosting.