1.Grease 2 deep 19cm square cake pans, line with paper, grease paper.
2.Melt chocolate over hot water, cool to room temperature.
3.Cream butter, essence and sugar in small bowl with electric mixer until light and fluffy. Add eggs one at a time, beating well after each addition. Transfer to large bowl, fold in sifted flour and milk in 2 lots.
4.Divide mixture into 2 bowls, stir chocolate into 1 bowl and orange rind into the other. Drop spoonfuls of chocolate mixture into the corners and centre of each cake pan. Spoon the orange mixture into the spaces. Swirl knife through mixture once or twice. Bake in moderate oven for about 35 minutes. Stand 5 minutes, before turning on to wire rack to cool.
5.To make Chocolate Fudge Frosting: Melt chocolate over hot water; cool to room temperature. Beat butter in small bowl with electric mixer until light and fluffy. Gradually beat in half the sifted icing sugar. Beat in chocolate, milk and essence, then remaining icing sugar; beat until frosting is spreadable.
6.Join cold cakes with about ¼ of the frosting. Spread top and sides of cake with remaining frosting.