Preheat oven to 180 °C. Sift cake flour, cocoa, and baking powder into a large bowl. Add sugar and salt. Whisk briefly to combine.
Add eggs, milk, vegetable oil and vanilla extract. Whisk well to combine until lump free - about 30 seconds.
Add boiling water and whisk to incorporate. Pour batter into cake pans.
Bake for 35 minutes. Set aside to cool for 10 minutes, or until cool completely before frosting.
For buttercream, place butter in the bowl of a stand mixer with paddle attached. Beat for 3 minutes on speed 7 until it becomes pale.
Add half the icing sugar and cocoa powder. Beat for 3 minutes - start slow, increase to speed.
Add remaining icing sugar and cocoa powder. Beat again for 3 minutes.
Add milk and vanilla if using. Beat again for 1 minute on speed 7.
Spread frosting onto cakes, or spoon into frosting bag and pipe.
1 1/2 teaspoon
Soft icing sugar