Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large mixing bowl, sift together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Beat on medium speed for 2 minutes until well combined.
Carefully stir in the boiling water. The batter will be thin, but this is normal.
Divide the batter evenly between the prepared cake pans.
Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cakes from the oven and allow them to cool in the pans for 10 minutes.
Turn the cakes out onto a wire rack and cool completely before frosting.
To make the chocolate buttercream frosting, beat the softened butter on medium speed until creamy, about 2 minutes.
Gradually add the powdered sugar and cocoa powder, beating on low speed until combined.
Add salt, vanilla extract, and heavy cream. Beat on high speed for about 3 minutes until light and fluffy.
If the frosting is too thick, add a little more cream (1 tablespoon at a time). If too thin, add more powdered sugar.
Place one cake layer on your serving plate. Spread a thick layer of frosting on top.
Place the second cake layer on top and apply a thin crumb coat of frosting all over the cake.
Chill the cake for 15 minutes, then apply the final layer of frosting smoothly over the entire cake.
Decorate as desired and serve.