Line the base of a 19cm (base measurement) spring-form tin with non-stick baking paper* (see note about tin size below). Process the biscuits in a food processor until finely crushed. Add the butter and process until well combined. Transfer to the prepared tin. Use a straight-sided glass to spread and press the biscuit mixture firmly over base and side of tin. Cover with plastic wrap and place in the fridge for 30 minutes to chill.
Preheat oven to 160°C (320°F).
Place a small saucepan with about an inch of water in it on the stove at a low heat until it simmers. Place a metal or glass bowl on top (ensuring the base of the bowl doesn’t touch the water) and add the chopped chocolate. Stir the chocolate as it melts until the mixture is smooth. Take it off the heat and set it aside to cool slightly.
Use an electric beater to beat the cream cheese, sugar and vanilla in a bowl until smooth. Add the eggs, 1 at a time, beating well after each addition until combined. Combine the cornflour and 2 tablespoons sour cream in a bowl. Add the cornflour mixture, remaining sour cream and chocolate to the cream cheese mixture and beat until combined.
Pour into biscuit base, leaving a good inch or so at the top, as the cake will rise. Place the tin on a baking tray with sides to catch any butter leakage**. Bake for 50-60 minutes or until the cheesecake is firm on the outsides, but still a little jiggly in the middle (it may even look a little damp in the middle – this is fine). Turn off oven. Leave the cheesecake in oven, with the door ajar, for 2 hours or until cooled completely (the cake will continue to cook and the centre will firm up in this time).
Take out of the oven and run a thin knife around the outside edges of the cake to prevent cracking as it cools. Place in the fridge still in its tin, with foil loosely covering it for at least 6 hours (or even better, overnight) to chill.
To make sour cherry sauce, combine cornflour and water in a bowl. Drain the cherries, reserving juice. Stir the cornflour mixture, reserved juice, sugar, vanilla bean and seeds, and star anise in a saucepan over medium heat for 5 minutes or until the mixture boils and thickens. Stir in the cherries. Set aside to cool slightly.
Take the cheesecake out of the fridge about 1/2 to 1 hour to serving time, remove from the spring-form tin and place the cake on a serving platter. Top with the sour cherry sauce when ready to serve.