Preheat your oven to 350°F (175°C). Grease the bottoms and sides of two 9-inch round cake pans and lightly dust with flour, or line the bottoms with parchment circles.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, unsweetened cocoa powder, baking powder, baking soda, and salt until evenly blended.

Add the eggs, whole milk, vegetable oil, and vanilla extract to the dry ingredients. With an electric mixer on medium speed, beat for about 2 minutes until the batter is smooth and well-combined.

Carefully stir in the boiling water by hand using a rubber spatula. The batter will become thin.

Divide the batter evenly between the prepared pans and smooth the tops with an offset spatula. Bake in the preheated oven for 30 to 35 minutes or until a toothpick comes out with a few moist crumbs.

Allow the cakes to cool in their pans for about 10 minutes, then carefully invert onto cooling racks to cool completely.

In a large bowl, beat the softened unsalted butter until light and creamy. Gradually add the sifted powdered sugar, mixing on low speed, then increase speed until smooth.

Add the melted dark chocolate and the heavy cream, and continue beating until glossy and spreadable.

Place one cooled layer on a cake plate or pedestal. Spread a generous layer of chocolate buttercream across the top. Add the second layer and frost the top and sides smoothly.

Slice with a sharp knife warmed under hot water and wiped dry for clean cuts. Enjoy immediately.