In a mixing bowl add flour, baking powder, salt, sugar, butter, eggs, vanilla, and milk.
Beat it well until the mixture is smooth.
Grease and flour a 9 inch round pan.
Reserve ¾ cup of batter; pour the rest into pan.
Preheat oven to 350 degrees F (175 degrees C).
In the reserved mixture, stir cocoa.
Drop the cocoa mixture with the help of spoon over top of white batter. Using a knife, swirl the cocoa batter into the white batter to incorporate it in a marble effect.
Bake in preheated oven for 30 to 35 minutes, until an inserted wooden pick comes out clean