Lightly grease a cake pan and cover the base with grease paper. Beat butter, essence and caster sugar in small bowl with electric mixture until it is light and fluffy. Add eggs one at a time, beating in well. Transfer mixture to a large clean bowl and fold in sifted flour and milk in 2 batches. Divide mixture in three bowls. Tint mixture in three bowls. Tint mixture pink in bowl 1, mix well. Stir in cocoa and extra milk in bowl 2 and mix well. Leave the third mixture plain. Drop spoonfuls of each mixture into prepared pan. Run a knife through cake mixture for a marble effect. If desired slip in a slab of dark bitter chocolate in the center so it melts while baking. Bake in a moderate oven about 40 minutes before turning onto a wire rack to cool.