Prepare the Base and Pan: Preheat your oven to 180°C (160°C fan). Grease and line the base of a 23cm (9-inch) springform tin with baking parchment. To prepare for the water bath, wrap the outside of the tin in a double layer of heavy-duty tin foil, ensuring it comes high up the sides to prevent any water from seeping in.

Make the Biscuit Base: Crush the digestive biscuits into fine crumbs using a food processor, or place them in a bag and bash with a rolling pin. Transfer the crumbs to a bowl and pour over the melted butter. Mix until it resembles damp sand. Tip this into your prepared tin and press down firmly and evenly with the back of a spoon. Bake for 10 minutes, then set aside to cool.

Create the Blackberry Swirl: While the base bakes, place the blackberries, sugar, and lemon juice in a small saucepan over a medium heat. Cook for 5-7 minutes, stirring occasionally, until the berries have broken down. Pour the cornflour slurry into the pan and stir continuously for another minute until the sauce thickens. Strain the sauce through a fine-mesh sieve to remove the seeds, pressing down on the solids to extract all the liquid. Set the smooth coulis aside to cool completely.

Start the Filling: In a large bowl using a stand mixer with the paddle attachment or a hand mixer, beat the room-temperature cream cheese on a low speed until completely smooth and free of lumps. This is important – start slow to avoid incorporating too much air. Scrape down the sides of the bowl.

Combine the Filling Ingredients: Add the caster sugar and beat on low until just combined. Add the soured cream and vanilla extract, mixing again until smooth. What works best for me is adding the eggs and yolk one at a time, beating on a low speed only until the yellow of each egg has disappeared into the batter. Do not overmix! Overmixing is the main cause of cracks.

Assemble the Cheesecake: Pour the cheesecake filling over your cooled biscuit base and smooth the top with a spatula. Dollop spoonfuls of the cooled blackberry coulis over the surface. Use a skewer or a thin knife to gently swirl the coulis into the filling to create a marbled pattern.

Bake in a Water Bath: Place your foil-wrapped tin into a larger roasting tray. Carefully pour boiling water into the roasting tray until it comes halfway up the sides of the cheesecake tin. This is the bain-marie, or water bath, and it ensures a gentle, even bake. Bake for 1 hour to 1 hour 15 minutes. The cheesecake is done when the edges are set but the centre has a slight wobble, like a jelly.

Cool Down Slowly: This step is key! Turn off the oven, prop the door open with a wooden spoon, and leave the cheesecake to cool in the oven for at least 1 hour. Then, remove it from the water bath, take off the foil, and let it cool to room temperature on a wire rack. Finally, cover and chill in the fridge for at least 6 hours, or preferably overnight.

Serve: Once fully chilled, carefully run a thin knife around the edge of the cheesecake before releasing the springform tin. Transfer to a serving plate, slice, and enjoy.