Preheat oven to 170 °Celsius.
Beat butter and sugar till soft and creamy for about 2 minutes. Add eggs, one at a time, beating well after each addition.
In a separate bowl, sift flour, baking powder, and salt. Fold in the sifted ingredients into the creamed mixture alternately with milk, starting and ending with flour.
Finally, add in vanilla extract and combine well. Pour batter into two greased and floured cake pans and bake for 45 to 50 minutes until the top of the cake springs back when lightly pressed. The cakes are done when a skewer inserted in the center of the cake comes out without any wet batter.
Remove cakes from oven and let them cool completely before icing and decorating.
Baking time may vary slightly for larger cakes.
To make the buttercream frosting, place butter in a large bowl. Beat until creamy.
Add the sifted powdered sugar and continue to beat until the icing turns light and fluffy. Add the vanilla extract and sifted black cocoa powder and beat again until the vanilla is well incorporated.
Add the black food coloring. The icing color will deepen when left to rest for about 20 minutes, so do not add too much coloring. Let the icing rest for 20 minutes and if the color is not deep enough, add more. If the icing is too stiff, add some milk and beat again.
To decorate the cake, fill the cake layers with buttercream. Use the remaining buttercream to cover the whole cake and smoothen it.