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 mixing bowl.
03 - Add milk, vegetable oil, eggs, and vanilla to dry ingredients. Beat on medium speed for 2 minutes until fully incorporated.
04 - Reduce mixer to low speed and carefully pour in boiling water. Mix until smooth. Batter will be thin in consistency.
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 before frosting.
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 to high and whip for 2 minutes until light and fluffy. Add additional milk if needed for spreading consistency.
09 - Place first cake layer on serving plate and spread frosting evenly over top. Position second layer and frost top and sides of entire cake. Decorate as desired.