Beat the softened butter with the sugar until you obtain a smooth and fluffy cream. Add the flour with vanilla spoon by spoon, the milk a little at a time and the egg.
Stir 1/2 the mixture with the cocoa. Combine the white and brown mixtures in an oiled and floured form.
Bake the cake in a preheated 356°F (180 °C) oven until ready.