Prepare the Tin and Base: Preheat your oven to 180°C (160°C fan). Grease and line the base of a 23cm springform cake tin with baking parchment. To make the base, crush the digestive biscuits into fine crumbs using a food processor, or place them in a bag and bash with a rolling pin. Mix the crumbs with the caster sugar and melted butter until it resembles wet sand. Tip into the prepared tin and press down firmly and evenly with the back of a spoon. Bake for 10 minutes, then set aside to cool.

Make the Raspberry Swirl: While the base cools, place the raspberries, 50g of sugar, and 1 tbsp of lemon juice into a small saucepan. Heat over a medium heat for 5-7 minutes, stirring occasionally, until the raspberries have broken down and the sauce has thickened slightly. For a smooth sauce, you can push it through a fine-mesh sieve to remove the seeds. I personally like the texture of the seeds, so I often leave them in. Set aside to cool completely.

Start the Cheesecake Filling: Reduce the oven temperature to 160°C (140°C fan). In a large bowl using a stand mixer or electric hand mixer, beat the room-temperature cream cheese on a low speed until smooth and creamy. Scrape down the sides of the bowl, then gradually add the 200g of caster sugar, beating until just combined. Don't over-mix here; we want to avoid incorporating too much air.

Add Wet Ingredients: Add the soured cream, vanilla extract, lemon zest, and lemon juice, and mix on low speed until just incorporated. Add the eggs and the extra yolk one at a time, mixing on the lowest speed until each one is just blended in. Scrape the bowl down well to ensure everything is combined. The final mixture should be smooth and velvety.

Assemble and Swirl: Pour the cheesecake filling over the cooled biscuit base. Dollop spoonfuls of the cooled raspberry sauce over the top of the filling. What works best for me when swirling is to use a skewer or the tip of a knife to gently drag the sauce through the batter in a figure-eight motion. Be careful not to overdo it, or you'll lose the distinct marbled pattern.

Prepare the Water Bath and Bake: Wrap the outside of your springform tin in a double layer of heavy-duty aluminium foil, ensuring it's watertight. Place the tin inside a larger roasting tray. Pour the cheesecake batter into the tin. Carefully pour boiling water into the roasting tray until it comes halfway up the sides of the cheesecake tin. This is called a bain-marie or water bath, and it's key to preventing cracks.

Bake and Cool Slowly: Carefully transfer to the preheated oven and bake for 60-70 minutes. The cheesecake is done when the edges are set, but the centre still has a slight wobble. Turn off the oven, prop the door open with a wooden spoon, and leave the cheesecake to cool completely in the oven for at least an hour. This gradual cooling process is another secret to a crack-free surface.

Chill Thoroughly: Once completely cool, remove the cheesecake from the water bath and the foil. Cover loosely with cling film and transfer to the refrigerator to chill for at least 6 hours, but preferably overnight. This step is non-negotiable for the perfect firm, sliceable texture. To serve, run a thin knife around the edge before releasing the springform tin.