Preheat oven to 170°C. Grease a 20cm round watertight cake pan and line with 1 piece of baking paper, folding excess over top of pan to make an even edge (using 1 whole sheet of baking paper will make it easier to remove the cooked cheesecake from the pan).

To make the base, place butter and biscuits in a food processor and whiz to combine. Press crumb mixture into base of the prepared pan. Chill until needed.

To make the filling, place cream cheese and sugar in a stand mixer fitted with the paddle attachment and beat until smooth. Add the cornflour and beat until combined. Add eggs, 1 at a time, beating well after each addition. Stir through lemon zest and juice, vanilla seeds and a pinch of salt.

Combine chocolate and sour cream in a bowl, then stir through cream cheese mixture. Pour ⅓ batter over base and scatter with ⅓ raspberries. Repeat layering process twice more, finishing by pressing final ⅓ raspberries into batter.

Place cheesecake in a large, deep baking dish. Fill dish with boiling water to halfway up side of cheesecake pan.

Bake for 1 hour 40 minutes or until cheesecake is firm to the touch with a slight wobble in the centre. Turn off oven. Stand cheesecake in oven with the door closed for 40 minutes, then remove pan from water bath and set cheesecake aside to cool to room temperature. Chill cooled cheesecake for at least 2 hours or until firm in the centre.

Once set, carefully lift from pan with baking paper. Slice and serve topped with freeze-dried raspberries, if using, or extra fresh raspberries.