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 with an electric mixer for 2 minutes until well combined.
Carefully stir in the boiling water. The batter will be thin; 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 wire racks and cool completely before frosting.
To make the 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 the salt, vanilla extract, and heavy cream. Beat on high speed for about 3 minutes until the frosting is light and fluffy.
Place one cake layer on a serving plate. Spread a layer of frosting on top.
Place the second cake layer on top and apply a thin crumb coat of frosting over the entire cake. Chill for 15 minutes.
Apply the remaining frosting evenly over the top and sides of the cake.
Slice and serve. Store leftovers covered at room temperature for up to 2 days or refrigerated for up to 5 days.