Preparation methodPreheat the oven to 180C/350F/Gas 4. Grease and line the base of a 23cm/9in spring form cake tin. Place the biscuits in a plastic bag and crush with a rolling pin to fine crumbs. Melt the butter and add the biscuit crumbs, stir to combine. Place in the base of the cake tin and spread in an even layer, then flatten (use your fingers or a potato masher). Bake in the oven for 10 minutes until golden. Remove and leave to cool while you prepare the filling. Reduce the oven to 160C/315F/Gas 2. In a large bowl, beat together the cream cheese and sugar until smooth then add the sour cream and flour and beat again. Gradually add the eggs and vanilla essence beating well between each addition (beat in rather than whisk as you don't want to incorporate too much air as this will affect the smooth surface of the cheesecake). Pour the cream cheese mix on to the biscuit base then bake in the oven for 45 minutes. The cheesecake should be just set with a slight wobble and should still be cream on top with just a slight golden hint around the edges. Once the cheesecake is cooked, turn off the oven and prop open the door so that it is slightly ajar and leave the cheesecake to cool in the oven – this prevents the top from cracking. Once the oven is cool you can remove the cheesecake to cool completely before removing from the tin.