1. Grease a deep 22cm-round cake pan; line base and side with baking paper.
2. Heat butter, hot water and sugar in a large pan. Stir over a low heat until sugar is dissolved. Transfer mixture to a large bowl; cool to warm.
3. Beat mixture with an electric mixer on low speed for 30 seconds. Beat in eggs, one at a time, until combined. Stir in sifted flours. Divide mixture in half. Stir white chocolate into one portion and dark chocolate into remaining portion.
4. Pour white portion into prepared pan, then dark portion. Use a skewer to drag through mixtures to create a marbled effect.
5. Cook in a slow oven, 150C, for one and a half hours or until cooked when tested. Cool cake in pan.
6. Ganache. Heat chocolate and cream in a small pan over low heat; stir until melted. Cool until a spreadable consistency.
7. Spread ganache over top and side of cake; decorate with chocolate shapes; serve with double thick cream.