Make the caramel by boiling the sugar and water in a saucepan, stirring continously over low-medium heat until the sugar is melted and becomes golden brown. Pour the syrup in a 9in round baking pan, tilting to make sure the whole surface is covered. Set aside to allow the caramel to harden.
Preheat the oven to 180ºC (350ºF). Gently combine all the custard ingredients in a separate bowl and whisk gently preventing any bubbles from forming. Strain gently into the caramel-lined baking pan.
Make the cake by beating the egg yolks with the sugar gradually, stirring until light and creamy. Add the flour alternately with the diluted milk, beginning and ending with flour. Add lemon rind and beat until smooth. Set aside. Whisk the egg whites and add the cream of tartar, beating to soft peaks. Add the sugar gradually and continue beating until stiff. Fold a third of the meringue into the cake mixture to loosen it. Gently fold in the remaining egg white mixture.
Pour gently the mixture into the prepared custard pan. Do not beat/stir. Place the cake pan in a bain-marie (bagno maria). Bake in the oven for an (1) hour or until the mixture is firm.
Remove from the oven and place on a wire rack to cool until room temperature. Remove from water to cool completely.
To serve, run a spatula or knife along the edge of the mold/pan to loosen the caramel. Transfer into a serving round dish and chill before serving.
