Heat the oven to 160°C. Oil and line a 9 in round cake tin (bottom and sides) with baking or parchment paper.
Melt butter, milk and cream cheese over a double boiler. Beat the egg yolks and lemon juice together.
Meanwhile sift together the cake flour, cornstarch and salt and mix well. Mix together all the mixtures - cream cheese, egg yolks and dry ingredients.
Whisk egg whites with cream of tartar until foamy. Add the sugar and beat until soft peaks form.
Spoon one third of egg white mixture into the cheese and egg yolk mixture at a time. Fold gently.
Pour into the prepared tin and bake in a water bath for 60- 70 minutes or until well risen and firm to touch. Leave the cake to cool down in the oven with the door open for 1 hour.
Transfer into the serving dish and rub in the margarine on top and sides of the cake. Dash with the icing sugar, if desired.
Refrigerate the cake for at least 6 hour. Overnight is the best.
