Line the base of an 8-inch/20-cm springform or loose-bottom cake pan with parchment. Mix the finely crushed biscuits with melted butter and the optional sugar, press firmly into the pan, and refrigerate for 10–15 minutes.

Beat the softened cream cheese and sugar on low to medium speed just until smooth. Add the eggs one at a time, then mix in the vanilla and sour cream until the batter is uniform, taking care not to whip in excess air.

Pour the filling over the chilled crust and smooth the top. Preheat the air fryer if the manufacturer's instructions require it, then cook at 160°C/320°F for about 35–45 minutes.

Check for a set edge and a slight wobble in the center, beginning to check at 30–35 minutes because air fryer models vary. Turn off the air fryer and leave the cheesecake inside with the basket or drawer slightly open for 15–20 minutes.

Cool the cheesecake completely, then refrigerate it for at least 4 hours or overnight. Remove it from the pan only when firm, slice with a warm dry knife, and serve chilled.