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