Preheat the oven to 350°F. Grease 2 (9-inch) round cake pans with butter or baking spray and line the bottom of each pan with parchment paper. (I highly recommend using cake strips for a more even bake.)
Sift the flour, sugar, cocoa, baking soda, baking powder, and salt into a large mixing bowl and whisk to combine.
In a medium mixing bowl, whisk together the milk, egg, oil, and vanilla. Add to the flour mixture and whisk together until well combined. Pour in the boiling water and whisk until well combined. (The batter will be runny.)
Divide the batter between the prepared cake pans.
Bake for 35 minutes or until a toothpick inserted in the center comes out clean. Let the cakes cool in the pans on a wire cooling rack for 15 minutes. Carefully invert the cake layers onto the wire rack, remove the parchment paper, and let cool completely.
For the Chocolate Buttercream:
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until soft and creamy, about 2 minutes. Add the cocoa powder and salt and mix on low speed until combined. Scrape the bowl down.
While mixing on low speed, gradually add in the powdered sugar alternating with a little bit of milk on occasion up to 4 tablespoons. Once the sugar is added, beat in the vanilla. Scrape down the bowl and beat on medium-low speed until light and fluffy, about 1 minute.
For the Assembly:
Place a cooled cake layer on a cake stand. Top with 1 cup of frosting and spread it out the edges in an even layer. Place the remaining cake layer on top and spread the rest of the frosting all over the outside of the cake. You can reserve some of the frosting to pipe decorations if desired.
Ingredient temperature matters. Use room temperature eggs and butter. Room temperature eggs give you a fluffier cake, and room temperature butter creams better for a feather-light buttercream frosting.
Measure the flour correctly. One of the top complaints when it comes to baking cakes is that the cake turns out “floury”, and this is due to using too much flour. The best way to get the right amount of flour is to measure it using a kitchen scale. Alternatively, you can spoon flour into a cup and then scrape a knife across to level it out. Scooping directly from the flour bag or container will inevitably lead to using too much of it.
Keep the cake moist for longer. For a cake that has even more moisture, substitute ⅓ cup of the whole milk for sour cream. This will give you a cake that will stay moist for longer, which is helpful if you plan to make it ahead or keep it for a few days after baking. Note that the batter will be a bit thicker than if you only used milk.
Amplify the chocolate flavor. To take the chocolatey-ness to the next level, use hot brewed coffee instead of water. Or add 1 tablespoon espresso powder to the wet ingredients. The cake will not taste like coffee, I promise! Coffee simply amplifies the flavor of the chocolate for a richer, more decadent cake.
Get perfectly flat layers every time. Cake strips are a must for flat, evenly baked, professional-looking cakes. (If you don’t have any on hand, they’re really easy to make with some aluminum foil and paper towels! Check out my easy DIY cake strip tutorial for more details.)