Break the dark chocolate and milk chocolate into equal pieces andput in a heatproof bowl. Heat the double cream in a small pan until just before boiling point. Pour the hot cream over the chocolate and leave to sit for at least 5 minutes.
Stir to combine until smooth.
If lumps of chocolate still remain, put the mixture back into the pan over a low heat and stir until smooth, or microwave briefly.
Let the ganache cool slightly in the bowl, stirring every few minutes until it forms a peanut-butter-like consistency. The cooling process can be sped up by placing the bowl in the fridge, but be sure to stir it frequently, so it doesn’t set too quickly.
Once the ganache has set slightly, it can also be whisked with a hand-held electric whisk to give it a lighter and more mousse-like texture.
Remove the chilled cake from the fridge and use a small angled spatula to smooth the ganache over the top and sides of the cake.
I use a large metal scraper to achieve an even finish.
Return the cake to chill in the fridge again for at least 30 minutes.