Preheat the 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 milk, vegetable oil, eggs, 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; 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. Then remove from pans and cool completely on wire racks.
To prepare the frosting, beat the softened butter on medium speed until creamy, about 2 minutes.
Gradually add the powdered sugar, cocoa powder, and salt, beating on low speed until combined.
Add the vanilla extract and milk. Beat on high speed for 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.
Decorate as desired and serve.