Steps:

• To make the base, whizz the biscuits in a food processor to fine crumbs, or put them in a sealed plastic food bag wrapped in a tea towel and bash with a rolling pin until crushed. Tip into a medium bowl and mix with the 25g sugar, the salt and melted butter (see Know-how). Press the mixture evenly into a greased 20cm loose-bottomed cake tin, then chill for at least 30 minutes or overnight.
• Next, make the filling: using a wooden spoon or wire whisk, mix the cream cheese, sugar and 200ml of the soured cream together in a medium bowl until smooth – don’t overbeat. Gently beat in the eggs, one at a time, then add the vanilla. Using a metal spoon, fold in the flour to form a smooth batter.
• Preheat the oven to 180°C/fan 160°C/gas 4. Remove the tin from the fridge and spoon the filling over the base. Put the tin on a baking sheet and bake for 45-60 minutes until the top is golden, set at the edges but still a bit wobbly in the middle. Leave to cool completely in the tin. When cool, transfer to the fridge to set for at least 3 hours or overnight.
• To serve, spread the remaining soured cream over the cheesecake in a thin, even layer.

Steps:

• Preheat 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.