1.Preheat oven to slow (150°C/130°C fan-forced). Grease two deep 20cm-round cake pans; line bases and sides with baking paper.
2.Combine butter, white chocolate, sugar and milk in medium saucepan; stir over heat, without boiling, until smooth. Transfer mixture to large bowl; cool 15 minutes.
3.Whisk sifted flours into white chocolate mixture then whisk in essence and egg; pour half of the mixture into one of the prepared pans. Whisk sifted cocoa into remaining mixture; pour into other prepared pan. Bake cakes, uncovered, about 50 minutes. Cool cakes in pans 5 minutes; turn cakes, top-side up, onto wire rack to cool.
4.Combine milk chocolate and cream in medium saucepan; stir over low heat until smooth. Transfer to medium bowl. Cover; refrigerate, stirring occasionally, until chocolate mixture is of spreadable consistency. Reserve 1 cup of the chocolate mixture for spreading over cake.
5.Split each cooled cake in half. Centre one layer of cake on serving plate; spread with ½ cup of the remaining milk chocolate mixture. Repeat layering, alternating colours. Cover top and sides of cake with reserved chocolate mixture.
You can also melt the milk chocolate and cream in a microwave oven; cook on HIGH (100%) about 90 seconds, pausing to stir every 30 seconds.