01 - Preheat oven to 350°F. Grease and flour two 9-inch round cake pans, lining 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 fully incorporated.
04 - Reduce mixer to low speed. Gradually add boiling water while mixing until batter is smooth and thin.
05 - Divide batter evenly between prepared pans. Bake for 30 to 35 minutes until a 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 in a large bowl until creamy. Add powdered sugar, cocoa powder, and salt. Mix on low speed until combined.
08 - Beat in vanilla and milk. Increase speed and whip for 2 minutes until light and fluffy. Adjust with additional milk if needed for spreadable consistency.
09 - Place first cake layer on serving plate. Spread frosting evenly over top. Position second layer and frost top and sides completely. Decorate as desired.