Preheat your oven to 350°F (175°C). Grease two 9-inch (22cm) round cake pans and line the bottoms with parchment paper.
In a large bowl, whisk together the all-purpose flour, cocoa powder, baking powder, baking soda, granulated sugar, and salt.
Add the eggs, whole milk, vegetable oil, and vanilla extract to the dry ingredients. Whisk well for about 30 seconds until the mixture is smooth and thick.
Carefully pour the boiling water into the batter. Whisk gently until the water is fully incorporated. The batter will be very thin.
Divide the batter evenly between the two prepared cake pans.
Bake for 35 minutes. A wooden skewer inserted into the center of the cakes should come out clean.
Let the cakes cool in the pans for 10 minutes.
Turn the cakes out onto wire racks and remove the parchment paper. Allow them to cool completely before applying any frosting.
Notes
If you only have one cake pan, you can bake the batter in batches. Keep the remaining batter at room temperature while the first layer bakes.
The batter is much thinner than standard cake batter. Do not add extra flour.
Wait until the cakes are entirely cool to the touch before frosting them. Warm cakes will melt the buttercream.