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1
Preheat the oven to 350u00b0 F. Get out a 10-inch flat-bottom tube or angel food cake pan. If the pan does not have a removable bottom, either line the bottom with parchment paper or grease it, then coat with matzo meal. If the pan does have a removable bottom, there's no need to do extra preparation.
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2
Combine the almonds, cake meal, potato starch, coffee powder, and chocolate in the bowl of a food processor fitted with the steel blade. Pulse until most of the almonds are finely ground (it's okay if some are a little coarser). Set aside.
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3
In a large bowl, whisk the egg yolks with half of the sugar just until blended. Grate the zest of the orange into the bowl. Whisk until the mixture is very thick and pale yellow.
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4
In the bowl of an electric mixer, beat the egg whites and cream of tartar (or vinegar) on medium-high speed until the mixture is creamy white and holds a soft shape. Gradually beat in the remaining 1/2 cup of sugar on high speed until the egg whites are stiff but not dry.
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5
Scrape 1/4 of the egg whites on top of the yolk mixture, and use a large rubber spatula to fold them in. Then scrape half of the remaining egg whites into the bowl along with half of the dry ingredients and fold until everything is almost blended. Repeat by adding the rest of the egg whites and dry ingredients and folding just until blended.
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6
Spoon the batter gently into the pan and smooth the surface. Bake 45 to 50 minutes, or until the cake springs back when lightly pressed and a toothpick inserted into the center comes out free of batter (though it may be coated with some melted chocolate).
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7
Set the pan on a rack to cool for 5 minutes. Slide a slim knife or small metal spatula around the sides of the cake to detach it from the pan, pressing against the pan sides to avoid tearing the cake. Detach the cake from the tube with the knife or a metal skewer. Leave the cake to finish cooling right side up, in the pan, on the rack.
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8
To unmold: If the pan sides are not removable, invert the cake onto a wire rack or a serving platter. Otherwise, pull upwards on the tube to lift the cake from the pan sides and slide the knife under the cake and around the tube to detach the bottom. Transfer the cake to a serving platter, right side or upside down, whichever you like. Dust a little powdered sugar over the top if desired, and serve with whipped cream.
