Line and grease the base and sides of a 9 inch springform tin. Wrap the outside of the tin with foil tightly so the foil comes up the sides of the tin.
Preheat oven to 160C or 310 F.
In a double boiler, add the cream cheese, milk and butter. On a low heat, stir until all melted, then set aside to cool.
Once cooled, lightly fold in the flour, cornstarch, salt, egg yolks, lemon juice and mix well.
In a separate bowl, whisk the egg whites with cream of tartar until frothy. Add the sugar, a little at a time until soft peaks.
Fold the whisked egg whites in batches into the cheese mixture gently until combined. Do not over mix or the cake will shrink when cooling. Pour the mixture into the tin.
Bake in a water bath for 1hr 10 mins until golden brown. *** During baking it is important NOT to open the oven door, to avoid surface cracking and collapse!
When the cook time is over, switch the oven off and leave cake to cool in the oven with door slightly open for a couple of hours. Once cooled, refrigerate for at least 4 hours or overnight to completely firm up before serving.