Step 1: Prepare the Biscuit Base: Finely crush the digestive biscuits, either in a food processor or by placing them in a resealable bag and crushing them with a rolling pin. In a bowl, combine the crushed biscuits with the melted butter and sugar. Mix well until the mixture resembles damp sand.
Step 2: Press into the Pan: Press the biscuit mixture firmly into the base of a 23cm (9-inch) springform pan. Ensure the base is evenly distributed and compacted. Chill in the refrigerator for at least 30 minutes to firm up.
Step 3: Prepare the Cheesecake Filling: In a large bowl, beat the softened cream cheese and sugar together until smooth and creamy. Be careful not to overmix, as this can introduce too much air into the mixture.
Step 4: Add Eggs and Yolks: Gradually add the eggs and egg yolks, one at a time, beating well after each addition. Ensure each egg is fully incorporated before adding the next.
Step 5: Incorporate Cream and Vanilla: Gently stir in the double cream, vanilla extract, and lemon zest (if using). Mix until just combined. Avoid overmixing to prevent a soupy batter.
Step 6: Bake the Cheesecake: Preheat the oven to 160C (320F). Wrap the base of the springform pan tightly with foil to prevent water from seeping in. Pour the cheesecake filling over the chilled biscuit base.
Step 7: Use a Water Bath (Optional but Recommended): Place the springform pan inside a larger roasting pan. Pour hot water into the roasting pan, reaching halfway up the sides of the springform pan. This creates a water bath, which helps the cheesecake bake evenly and prevents cracking.
Step 8: Bake: Bake in the preheated oven for 50-60 minutes, or until the cheesecake is mostly set but still has a slight wobble in the center. If the top is browning too quickly, tent it loosely with foil.
Step 9: Cool and Chill: Turn off the oven and crack the door slightly. Allow the cheesecake to cool in the oven for 1 hour. This gradual cooling helps prevent cracking. Remove the cheesecake from the water bath and let it cool completely at room temperature.
Step 10: Chill Completely: Cover the cheesecake with plastic wrap and refrigerate for at least 6 hours, or preferably overnight, to allow it to set completely.
Step 11: Serve: Before serving, run a thin knife around the edge of the cheesecake to loosen it from the pan. Gently release the springform and transfer the cheesecake to a serving plate. Slice and serve.
Optional Tips: For a richer flavor, use brown sugar in the biscuit base. To prevent a soggy base, brush the biscuit base with melted chocolate before adding the filling. For a more pronounced vanilla flavor, use vanilla bean paste instead of extract. If you don't have double cream, heavy cream can be used as a substitute.