Preheat the oven to 325 degrees F. Set a non-greased 10" angel food cake pan to the side.

In a food processor, process the granulated sugar for about 2 minutes or until finely ground. Pour half of the sugar into a large bowl and add the flour. Set the remaining sugar aside.

Using your mixer, beat the eggs whites and salt until foamy. Add the cream of tartar and beat until peaks start to form. Gradually, 1 tablespoon at a time, add the reserved finely ground sugar to the egg whites, beating until soft, shiny, droopy peaks form. Mix in the vanilla extract.

Scatter one-third of the sugar and flour mixture over the egg whites and gently fold in. Repeat with the remaining sugar-flour mixture. Fold in the orange zest. Be very careful to not over mix. This will cause the cake mixture to loose too much air and become heavy once baked.

Spoon the batter into the cake pan. Bake for 1 hour, or until golden brown and the cake is set.

Remove from the oven and immediately flip the pan upside own on it's feet. If you pan does not have any feet, invert it onto the neck of a bottle or an upside down funnel. Let stand until cool, then run a knife or metal spatula around the sides and the center tube of the pan. Run the knife around the bottom to release the cake.

In a double boiler or a large bowl set over, not in, a pan of simmering water, whish together the whole eggs, egg yolks, lemon juice, sugar, and butter. Cook over low heat, whisking constantly, for 5 minutes or until the mixture has the consistency of honey. Using a fine mesh strainer, strain the mixture into a bowl.

Cool to room temperature and cover. Place in the fridge and store up too 3 weeks.