Preheat the oven to 180°C/gas mark 4. Line the base and sides of a 20cm x 30cm x 5cm baking tray with baking paper. Beat the butter and sugar in a large mixing bowl with a hand-held electric whisk until smooth and creamy.
Gradually beat in the eggs, a little at a time, until well combined. If the mixture starts to curdle, add 1 tbsp flour. Sift in the remaining flour and cocoa powder then, using a wooden spoon, carefully fold in the milk. Spread the mixture evenly in the tray. Bake for 25 minutes until the cake springs back to the touch and a skewer inserted into the middle comes out clean. Remove from the tray and cool on a wire rack.
Meanwhile, melt the dark chocolate in a heatproof bowl set over a pan of simmering water. Once melted, take the bowl off the pan and beat in the cream until smooth and glossy.
Spread the ganache over the cooled cake. Leave for a few minutes to set, then cut into 24 squares. Melt the white chocolate in a bowl in the microwave on high for 30 seconds. Spoon the melted white chocolate into a piping bag and pipe lines over the cake. Add your choice of decorations on top.