Preheat the oven to 180°C, fan 160°C, gas 4. Line the base and sides of a 20cm x 30cm x 5cm baking tray with baking paper. In a large mixing bowl, beat the butter and sugar 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 tablespoon of the flour. Sift in the 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, make the ganache: melt the dark chocolate in a heatproof bowl set over a pan of simmering water (don’t let the bowl touch the 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. Decorate with the rose and blossom cake decorations and a few mint leaves, if using.