01 - Preheat the oven to 350°F. Grease and flour two 9-inch round cake pans, then line bottoms with parchment paper.
02 - In a large bowl, sift together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
03 - Add milk, vegetable oil, eggs, and vanilla to the dry ingredients. Beat on medium speed until well combined, about 2 minutes.
04 - Reduce speed to low and carefully add boiling water to the batter. Mix until smooth. The batter will be thin.
05 - Divide batter evenly between prepared pans.
06 - Bake for 30 to 35 minutes, or until a toothpick inserted into the center comes out clean.
07 - Cool cakes in pans for 10 minutes, then turn out onto a wire rack to cool completely.
08 - In a large bowl, beat softened butter until creamy. Add powdered sugar, cocoa powder, and salt; mix on low until combined.
09 - Beat in vanilla and milk. Increase speed and beat until light and fluffy, about 2 minutes. Add more milk if needed for spreading consistency.
10 - Place one cake layer on a serving plate. Spread frosting over the top. Place the second layer on top and frost the top and sides of the cake.
11 - Decorate as desired.