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1
Position a rack in the lower third of the oven and preheat the oven to 325u00b0 F. If your tube pan does not have feet, locate a bottle or something that will allow you to suspend and cool the cake upside down.
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2
Put 1 cup (200 grams) of the sugar and the salt in a large bowl. Grate the zest of one orange and the lemon over the sugar.
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3
Squeeze, and set aside, enough of the orange to make 6 tablespoons of juice. In a separate cup, squeeze enough of the lemon to make 2 teaspoons of juice.
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4
Add the egg yolks to the sugar and beat with the handheld mixer until very light and thick. (If you have arm strength and stamina, you can do this by hand with a whisk.) Beat in the orange juice, tiger nut flour, and starch. Set aside.
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5
Put the eggs whites and lemon juice in the bowl of the stand mixer. Beat with the whisk attachment on medium-high speed until the whites are creamy white (no longer translucent) and hold a soft shape. Slowly sprinkle in the remaining 1/4 cup sugar, beating on high speed until the egg whites are stiff but not dry.
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6
Scrape 1/3 of the egg whites on top of the yolk mixture and fold until almost completely blended. Fold in the remaining egg whites in two additions.
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7
If using cinnamon, scrape about one third of the batter into the pan and sprinkle pinches of half of the cinnamon over it. Cover with half of the remaining batter and sprinkle with the rest of the cinnamon. Top with the remaining batter. If not, scrape all of the batter into the pan. Tilt the pan to level the batter. Bake until the top of the cake is golden brown and springs back when gently pressed (about 30 minutes). A wooden skewer inserted into the cake should come out clean. Invert the pan over a bottle and let it cool for at least two hours before setting right side up.
