Prepare the sweetcorn: Preheat the oven to 180 C / 160 C fan. Put the sweet corn cobs onto a baking sheet and roast for 30 minutes. Once cool enough to handle, cut off the kernels and put into a saucepan with the sour cream. Bring to a gentle simmer then take off the heat and allow to infuse for 1 hour. Pour into a food processor and blitz, then strain and put in the fridge until needed.
Make the base: Preheat the oven to 180 C (160 C fan), and line the bottom and sides of a 22 centimeter springform tin with greaseproof paper. Blitz the biscuits and roasted corn into a food processor until finely ground, then tip in the melted butter and process until it’s just mixed. Tip the crumb into the prepared cake tin and press it down with a glass or the back of a spoon, across the bottom and up the sides to the top of the tin, so that it’s smooth and compacted. Put the tin into the fridge while you make the filling.
Wrap two layers of foil securely around the outside of the cake tin - covering the bottom and up the sides, to prevent any water seeping in. Put the tin into a deep oven tray or dish, to act as a water bath, and boil the kettle.
Make the filling: Put the sweet corn cream into a large mixing bowl, and use an electric hand whisk to beat in the cream cheese and sugar. Add the eggs and egg yolks and mix until smooth.
Pour the cheesecake filling into the tin and cover the top of the tin loosely with foil, then put the whole thing into the oven. Pour boiling water into the water bath, so that it comes half-way up the cheesecake. Bake the cheesecake for 1hr 30 minutes, until just set. Switch off the oven and leave the cheesecake in there for 1 hour, so it cools slowly and is less likely to crack. Then remove the cake tin from the water bath and leave it to cool completely, in its tin, before putting in the fridge for at least 4 hours, or overnight.
Serving the cheesecake: Run a hot pallet knife around the edge of the cheesecake to loosen it from the edge of the tin. Carefully transfer it to a serving plate, decorate with the popcorn, and serve.