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