01 - Preheat oven to 350°F. Grease and flour two 9-inch round cake pans, then line bottoms with parchment paper.
02 - Sift together flour, sugar, cocoa powder, baking powder, baking soda, and salt in 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 speed to low. Carefully add boiling water and mix until smooth. The 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. Turn out onto wire rack and cool completely before frosting.
07 - Beat softened butter until creamy. Add powdered sugar, cocoa powder, and salt. Mix on low until combined.
08 - Beat in vanilla and milk. Increase speed and beat for 2 minutes until light and fluffy. Add more milk if needed for spreading consistency.
09 - Place one cake layer on serving plate. Spread frosting over top. Add second layer and frost top and sides. Decorate as desired.