23cm springclip tin, greased and base-lined, set on a baking tray
Heat the oven to 150C/300F/Gas 2. Make the base first: mix the cocoa with the melted butter then stir in the biscuit crumbs and the sugar. When thoroughly combined tip the mixture into the tin and, using the back of a spoon, press firmly onto the base and about 2cm up the sides. Chill until needed.
Break up the white chocolate and melt gently with the water. Remove the bowl from the heat, stir gently and leave to cool until needed.
Put the cream cheese, sugar, orange zest, lemon juice and zest, vanilla and eggs into the bowl of a food processor. Run the machine until the mixture is very smooth, scraping down the sides from time to time. Add the melted chocolate and run the machine until thoroughly combined. Pour the mixture into the chilled base, then set the tin (on the baking tray) into the heated oven and bake for 40 minutes.
Once cooked turn off the oven but do not remove the cheesecake and leave to cool for an hour. Then remove the tin, leave to cool completely then cover and chill overnight. When ready to serve, run a round-bladed knife inside the tin to loosen the cheesecake then unclip the tin.
Decorate with plain chocolate shavings or dust with cocoa. Store in an airtight container in the fridge and eat within 4 days.