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Preheat the oven to 325 degrees F. Set a non-greased 10" angel food cake pan to the side.
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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.
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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.
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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.
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Spoon the batter into the cake pan. Bake for 1 hour, or until golden brown and the cake is set.
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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.