Butter two cake tins, 24cm and 18cm, and line the bases with parchment paper. Heat oven to 180C/fan 160C/gas 4. Beat the butter and sugar in a bowl until light and creamy. Gradually beat in the eggs and orange zest, adding some flour with the last few additions to prevent curdling. Using a metal spoon, fold in the remaining flour and milk. Split the mix between the tins and smooth the tops.
Bake the cakes together, 30-35 mins for the smaller and 40-45 mins for the larger, until golden and springy. Cool in the tins, then turn out on a rack to cool fully.
Break milk and dark chocolate into a bowl and melt over a pan of simmering water, stirring occasionally. Melt white chocolate the same way. Peel the paper from the cakes and sit the largest on a plate. Spread a small blob of chocolate in the middle and sit the smaller cake on top. Drizzle with chocolate, sprinkle with edible gold or silver balls and leave to set.
Keep in an airtight container.