Preheat oven to 170 °Celsius. Grease and line two 7-inches round cake tins.
Mix cocoa powder with ¼ of the milk and leave aside.
In a separate bowl, beat butter and sugar till soft and creamy.
Add eggs, one at a time, beating well after each addition.
In another 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 essence and combine well.
Transfer ⅓ of the batter into a separate bowl and add the milk-cocoa mixture to it. Mix well.
Scoop the plain batter and chocolate batter alternately into 2 equal sized, well greased and floured cake tins.
Bake the cakes for approximately 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 frosting and decorating.
Baking time may vary slightly for larger cakes.
To make the buttercream, cream butter in a large bowl. Add the sifted icing sugar and beat until the buttercream turns fluffy.
Remove a small amount of the buttercream into a separate bowl. Mix in the sifted cocoa powder. Add some milk if the icing is too stiff.
To assemble the cake, fill each cake layer with the plain buttercream. Use the remaining icing to cover the sides and top of the cake. Smooth the buttercream.
Use a spatula to spread the chocolate buttercream in random intervals on the sides and top fo the cake. Use an icing scraper to smooth the buttercream on the cake.
Use a palatte knife to create horizontal lines on the sides of the cake and circular patterns on the top of the cake.