01 - Preheat oven to 350°F. Grease two 9-inch round cake pans and line bottoms with parchment paper.
02 - In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt until well blended.
03 - Add eggs, milk, oil, and vanilla to the dry mixture. Beat on medium speed until fully combined, about 2 minutes.
04 - Carefully stir in the boiling water. The batter will become thin—this is normal for a moist cake texture.
05 - Divide batter evenly between prepared pans. Bake for 30–35 minutes until a toothpick inserted in the center comes out clean.
06 - Let cakes cool in pans for 10 minutes, then invert onto wire racks to cool completely before frosting.
07 - Beat softened butter in a large bowl until creamy. Sift in cocoa powder and mix until fully incorporated.
08 - Gradually add powdered sugar, alternating with milk, beating until smooth and spreadable. Mix in vanilla and salt.
09 - Place one cooled cake layer on a serving plate. Spread an even layer of frosting over the top.
10 - Stack the second cake layer. Frost the top and sides with remaining frosting, using an offset spatula to create swoops and swirls.