To make the base use a food processor to whizz up the digestive biscuits into crumbs, or gently crush them with a rolling pin in a bowl
Mix in the melted butter into the biscuit crumbs and press the mixture into the bottom of a 23cm springform tin. Put it in the fridge for 30 minutes to chill
For the cheesecake filling use a food mixer with a whisk attachment or an electric hand whisk to mix together the cream cheese, icing sugar and cocoa powder until smooth with no lumps
Add the double cream and whisk until it is very thick and holds it's shape
Smooth the mixture into the tin on top of the biscuit base, then put it in the fridge overnight, or for at least 4 hours, to set
Remove from the tin and put on your serving plate. Smooth the sides with a butter knife to create a neater finish
To make the dark chocolate ganache, put the ingredients into a bowl and microwave for 20 seconds, then stir. Continue microwaving in 10 second blasts, stirring between each blast, until melted and smooth
Spread the dark chocolate ganache over the top of the cheesecake
To make the white chocolate ganache, put the ingredients into a bowl and microwave for 20 seconds, then stir. Continue microwaving in 10 second blasts, stirring between each blast, until melted and smooth. If you are using the whitening powder, sieve it into the melted mix and stir in
Put the white chocolate ganache into a piping bag and pipe a swirl, starting from the centre of the cheesecake and working your way out
Using a cocktail stick, starting from the centre of the cheesecake and working your way to the edge, drag the cocktail stick through the ganache to create the spiderweb pattern
Serve immediately, store any leftovers in the fridge and eat within 3 days