Preheat the oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or line them with parchment paper.
Mix dry ingredients: In a medium bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
Mix wet ingredients: In a large mixing bowl, beat together sugar, eggs, milk, oil, and vanilla extract until smooth and combined.
Add dry ingredients to wet ingredients: Gradually add the dry ingredient mixture into the wet ingredients. Mix until smooth. The batter will be thick.
Add boiling water: Slowly pour the boiling water into the batter. Mix until well combined. The batter will be thin, but that’s okay—it ensures the cake stays moist!
Bake: Divide the batter evenly between the two prepared pans. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Cool the cakes: Let the cakes cool in the pans for 10 minutes before removing them from the pans. Cool completely on a wire rack before frosting.
Make the frosting: In a large mixing bowl, beat the softened butter until creamy. Gradually add powdered sugar, cocoa powder, and salt. Add milk or heavy cream a little at a time, and beat until smooth. Stir in vanilla extract.
Assemble the cake: Once the cakes have cooled, frost the top of one cake layer. Place the second layer on top, and frost the top and sides of the cake evenly with the frosting.
Serve and enjoy! Slice and serve the cake at room temperature for the best texture.
