01 - Preheat oven to 350°F. Grease and flour two 8-inch round cake pans, then line with parchment paper.
02 - Sift together flour, sugar, cocoa powder, baking powder, baking soda, and salt in a large mixing bowl.
03 - Add eggs, milk, oil, and vanilla extract to the dry mixture. Beat on medium speed until smooth and fully combined, about 2 minutes.
04 - Carefully pour in boiling water while mixing on low speed. Continue mixing until batter becomes glossy and thin, about 1 minute.
05 - Divide batter evenly between prepared pans. Bake for 30-35 minutes until a toothpick inserted in center comes out clean.
06 - Let cakes cool in pans for 10 minutes. Run a knife around edges, then turn out onto wire racks to cool completely before frosting.
07 - Beat softened butter in a large bowl until creamy and smooth, about 2 minutes. Gradually sift in powdered sugar and cocoa powder, beating until fully incorporated.
08 - Add vanilla extract and salt. Beat in milk one tablespoon at a time until frosting reaches smooth, spreadable consistency.
09 - Place one cooled cake layer on serving plate. Spread frosting evenly over top. Position second layer on top, then frost top and sides completely.
10 - Smooth frosting with spatula. Decorate as desired. Slice and serve immediately or refrigerate until ready to serve.