Preheat oven to moderately slow.Grease deep 20cm-round cake pan; lin base and side with baking paper. Combin butter, chocolate, sugar and milk in medium sascepan;using wooden spoon, stit over low heat,without boiling,until smooth.Transfer mixture to large bowl; cool 15 minutes. Whisk in flours then essence and egg;pour mixture into prepared pan. Bake in moderately slow oven 1 hour. Cover pan loosely with foil; bake about 1 hour.Discard foil, stand cake in pan 10 miutes then turn onto wire rack; turn top- side up to cool. Place cake on serving plaste, spread all over with white chocolate ganache. White chocolate ganache Bring cream to a boil in small sucepan; pour ovre chocolate in small bowl, stir with wooden spoon until chocolate melts.Cover bowl; refrigerate, stirring occasionally, about 30 minutes or until ganache is of a spreadable consistency.