Special equipment: 10 by 4-inch tube pan or 9 to 10-inch round cake pan with sides at least 3 inches high (a removable bottom is helpful)
For the raspberry sauce, mix 1 cup of the raspberries, the preserves, water, and the sugar in a medium-size heavy saucepan (not uncoated aluminum.) Bring to a boil over moderately high heat, mashing the raspberries with a wooden spoon. Boil 1 minute. Remove from the heat. Strain the raspberry mixture through a fine strainer suspended over a bowl, pressing down on raspberries to release the juices. Discard the seeds. Stir the blueberries and the remaining 1 cup raspberries into the hot raspberry syrup. Cover and let stand until cool. Refrigerate until ready to use. Yield: 2 cups
For the cake, check that one rack is in the middle of the oven and heat the oven to 325 degrees. Pour the egg whites into the bowl in which you will be making the batter. If the pan has a removable bottom, do nothing to it. If it does not, lightly butter the bottom. Line the bottom with waxed paper or parchment paper cut to fit, and lightly butter the paper. Thoroughly mix the flour and 1/4 cup of the granulated sugar or 1/2 cup of the confectioners' sugar in a small bowl. Have all the ingredients and tools at hand. Beat the egg whites with an electric mixer on medium speed for about 2 minutes, until frothy and well broken up. Add the cream of tartar and increase the speed to medium-high. Beat until the whites lose their yellow cast, greatly increase in volume, and start to turn white. With the mixer running, slowly sprinkle the remaining 3/4 cup sugar over the whites. Beat until the whites become very thick, very glossy, and white, and the beaters leave a deep trail. Depending on the mixer, this need take only about 3 minutes total. Beat in the lemon juice and vanilla, just enough to mix. Reduce the speed to the lowest possible. Quickly sprinkle the flour sugar-mixture over the whites. As soon as it is all added, but not completely mixed in, stop the machine and remove the bowl. With a rubber spatula or a large metal spoon, complete mixing in the flour by folding or gently stirring. Stop just as soon as the flour seems blended in. With a rubber spatula, scrape the batter (9 cups) into the pan and spread evenly. Inscribe a circle deep in the batter to release any large air bubbles. Bake until no moist patches remain in the surface cracks, the cake springs back when touched, and a cake tester inserted in the center comes out clean, about 50 minutes for a 10 by 3-inch pan, 1 hour for a tube pan. Turn the pan upside down on a wire rack. Let cool completely. Loosen the edges (and tube) with a knife. Turn out the cake, loosen and remove the pan bottom, or peel off the paper. Store airtight 1 day at room temperature before serving or freezing. When serving, top cake with sauce and confectioners' sugar, if you wish.
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