Blueberry sauce: In a small saucepan, cook the blueberries with the sugar on a medium heat until they have released their juices and part of the liquid has evaporated.
After about 8-10 minutes, remove from the heat and allow to cool completely.
Preheat the oven to 180°C fan / 400°F / Gas 6.
Biscuit base: Melt the butter in a small saucepan (or in the microwave).
Put the biscuits in a bag and tap them with a rolling pin or place them in a food processor and pulse them into fine crumbs. Place the crushed biscuits into a medium bowl and mix in the melted butter. It should resemble wet sand. Tip this mixture into a lined 20cm (8 inches) square baking pan or round springform, spread it out evenly and pack down firmly with the base of a glass.
Bake the base in the preheated oven for 10 minutes then remove and leave to cool. Reduce the temperature of the oven to 120°C fan / 275F / Gas 1.
Cheesecake filling: In a large bowl, mix the cream cheese and yoghurt together until smooth. (Avoid over mixing – you don't want to incorporate too much air into the mixture.)
Add the sugar, salt, vanilla and cornstarch, and mix until combined.
Add the eggs, one at a time, mixing well after each addition, until smooth.
Add 1.5 tablespoons of the cheesecake filling to the cooled blueberry sauce and mix together. (This will stop the blueberries in the sauce from sinking.)Pour the rest of the cheesecake filling on top of the cooled biscuit base and smooth it out evenly. Gently spoon the blueberry sauce on top of the cheesecake filling and using a skewer or toothpick, swirl it around to create a pattern.
Baking: Place the cake onto the middle shelf of the oven and bake for 30-35 minutes until the middle of the cake is still slightly jiggly when the tray is shaken.Turn off the oven and allow the cheesecake bars to cool to room temperature with the oven door ajar. Once cooled to room temperature, transfer the cheesecake bars to the fridge for an hour or 2 two to set.
Serve as they are or with whipped cream.