01 - Preheat the oven to 350°F. Grease, flour two 9-inch round cake pans, and line the bottoms with parchment paper.
02 - In a large bowl, whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt.
03 - Add eggs, milk, vegetable oil, and vanilla extract. Beat on medium speed for 2 minutes until smooth.
04 - Carefully stir in boiling water; the batter will be thin.
05 - Divide batter evenly between the prepared pans.
06 - Bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out clean.
07 - Allow cakes to cool in pans for 10 minutes; then remove from pans and cool completely on wire racks.
08 - Beat softened butter until creamy. Add powdered sugar, cocoa powder, and salt; beat until combined.
09 - Add vanilla extract and milk, beating until light and fluffy. Add additional milk as needed to achieve spreadable consistency.
10 - Place one cake layer on a serving plate, spread with frosting, top with second layer, and frost the top and sides evenly.