01 - Preheat oven to 350°F. Grease and dust two 9-inch round cake pans with flour.
02 - Whisk flour, cocoa powder, sugar, baking powder, baking soda, and salt together in a large bowl.
03 - Add eggs, whole milk, vegetable oil, and vanilla extract to the dry mixture. Beat on medium speed until smooth, about 2 minutes.
04 - Carefully stir in boiling water; batter will be thin.
05 - Divide batter evenly between prepared pans and bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out clean.
06 - Allow cakes to cool in pans for 10 minutes before transferring to wire racks to cool completely.
07 - Beat softened butter until creamy. Sift in powdered sugar and cocoa powder. Add salt and vanilla extract, then beat until combined. Gradually add milk while beating until the frosting is smooth and spreadable.
08 - Place one cake layer on a serving plate and spread with frosting. Top with the second cake layer and frost the top and sides evenly.
09 - Slice and serve promptly.