Makes 12 servings

For the cake:

• 2 cups all-purpose flour
• 1 teaspoon baking powder
• 1 teaspoon baking soda
• 2 sticks butter, at room temperature
• 1 cup sugar
• 4 egg yolks
• 4 egg whites
• 1 cup sour cream
• 1 tablespoon minced orange peel
• 1 tablespoon minced lemon peel
• Pinch of cream of tartar

To make the syrup:

• ¾ cup sugar
• ¼ cup orange-flavored liqueur
• ¼ cup fresh orange juice
• 3 tablespoons fresh lemon juice
• Pinch of salt
• 6 thin lemon slices
• 5 thin orange slices
• 1 cup whipping cream, whipped

To make the cake, preheat the oven to 325 degrees.  Butter and flour a 9-inch angel food cake pan. Sift together the flour, baking powder and baking soda.  Using an electric mixer, cream the butter and sugar.  Add the yolks, sour cream, and orange and lemon peels.  Beat until slowly dissolving ribbons forms when beaters are lifted.

Stir in the dry ingredients.

Using clean beaters, beat the egg whites with cream of tartar until stiff but not dry.  Gently fold half of the whites into the batter to lighten the mixture, then fold in the remaining whites.  Pour into the prepared pan.  Bake until tester inserted in the center comes out clean, about one hour. Remove from the oven and let cool in the pan for 15 minutes.  Run a knife around the edges of the cake to loosen and invert onto a platter.

To make the syrup, cook the sugar, orange liqueur, orange and lemon juice, and the salt in a heavy saucepan over low heat until the sugar dissolves, swirling the pan.  Increase the heat and boil until the mixture is thick and syrupy.

Using a toothpick, pierce the top of the cake all over.  Pour the syrup over the cake.  Garnish with the lemon and orange slices.  Serve with whipped cream.