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1
Preheat the oven to 325 and position a rack in the lower third.
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2
In a bowl, whisk the cake flour with 1 cup of the granulated sugar, the baking powder and salt.
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3
Make a well in the center and add the oil, egg yolks, zests, extracts and grapefruit juice.
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4
Slowly whisk the liquids into the flour, then whisk vigorously until smooth, about 30 seconds.
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5
In a bowl, using an electric mixer, beat the egg whites at medium speed until frothy.
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6
Add the cream of tartar and beat until very soft peaks form.
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7
Gradually add the remaining 1/2 cup of granulated sugar and beat until glossy but not stiff, about 1 minute.
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8
Fold one-fourth of the beaten whites into the batter, then fold in the rest.
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9
Scrape the batter into an ungreased 10-by-4-inch angel food cake pan (not nonstick) with a removable bottom.
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10
Bake the cake for 50 minutes, or until a toothpick inserted in the center comes out clean.
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11
Invert the cake onto the neck of a wine bottle; let cool.
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12
Run a knife around the inside of the pan and remove the outer ring.
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13
Run the knife around the tube and along the bottom and remove the tube.
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14
Cut the cake into wedges and sift with confectioners' sugar.
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15
Serve with the frozen yogurt and oranges.
