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1
Position a rack in the lower third of the oven. Preheat the oven to 325u00b0 F. Grease a 10-inch angel food cake (plain tube) pan with a removable bottom with vegetable oil spray or butter.
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2
In a food processor with a dry bowl and blade, pulse the nuts until finely ground.
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3
In a large mixing bowl whisk the egg yolks, salt, vanilla, and 1/3 cup of the sugar just to blend. Grate the zest of the lemon directly into the bowl. Whisk until the mixture is thick and pale yellow.
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4
Combine the egg whites and cream of tartar in the clean dry bowl of a stand mixer (or other large bowl if using a hand-held mixer) and beat with the whisk attachment on medium-high speed (or high speed with a hand mixer) until soft peaks form when the whisk is lifted. Gradually add the remaining 1/4 cup of sugar, at high speed, until peaks are stiff but not dry when the whisk is lifted.
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5
Scrape half of the egg whites over the yolks and pour half of the nut flour on top. Use a large rubber spatula to fold until the elements are partially incorporated. Repeat with the remaining egg whites and nut flour, folding just until incorporated. Scrape the batter into the pan (it will be slightly less than half full) and spread it evenly. Bake until the cake is golden brown on top, springs back when lightly pressed, and a toothpick inserted in the center comes out clean, 30 to 35 minutes. Set the pan on a rack to cool.
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6
To unmold the cake, run a skewer around the tube and slide a thin spatula around the sides of the pan to detach the cake. Lift the tube to remove the cake. Slide the spatula under the cake all around to detach the bottom. Use two spatulas to lift the cake off of the bottom. Serve the cake upside down or right side up! It keeps, wrapped airtight, at room temperature for at least 3 days, or in the freezer for up to 3 months; bring to room temperature before serving.
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7
Sieve a little powdered sugar or the cake before serving, if desired. Serve plain or with berries and cream.
