For the Cake
Preheat oven to 350F. Line the bottom of three ungreased 9-inch round cake pans with parchment. Do not grease.

Sift together the flour, 1 cup of the sugar, and the baking soda. Set aside.

In a large clean mixer bowl, whip the egg whites with the cream of tartar until frothy with an electric mixer on high speed. Slowly add the remaining 1 cup of sugar and continue whipping until soft peaks form.

In a large bowl, whisk together the buttermilk, oil, and vanilla until well blended. Spoon one-fourth of the whipped egg whites on top of the buttermilk mixture, then sift a third of the dry ingredients on top.

Fold gently until just a few streaks remain. Repeat two more times, then add the remaining egg whites and fold in gently. Divide the batter among the 3 prepared cake pans.

Bake for 20-22 minutes, until a cake tester or wooden toothpick stuck into the center of each layer comes out clean.

Transfer to wire racks and let the cakes cool completely in their pans, at least 1 hour, before turning out. Run a blunt knife around the rim of the pan. Carefully peel off parchment.

For the Cranberry-Raspberry Preserves
Place all ingredients except vanilla bean into a heave non-reactive pan. Split the vanilla bean lengthwise without cutting all the way through. With the tip of a small knife, scrape the vanilla seeds into the saucepan. Toss in bean pod as well.

Bring to a boil over medium heat, stirring to dissolve the sugar. Reduce the heat slightly and continue to boil until preserves visibly thickens, 15-20 minutes.

Remove and discard vanilla bean pod. Let the preserves cool completely, then cover and refrigerate until ready to use. They will keep well for up to 2 weeks. (You will have ~1/2 cup more than necessary for the cake).

For the White Chocolate Frosting
Melt the white chocolate in a microwave oven on low power for 20-30 seconds, until the chocolate is soft and shiny. It will not look melted until your stir it. Set it aside to cool so it doesn't melt the butter.

In a small heavy saucepan, combine the sugar and water to make a thick paste. Bring to a boil, stirring to dissolve the sugar. Continue to cook over med-low heat without stirring, until the syrup reaches 238F on a candy thermometer.

Put the egg whites into a large mixer bowl, set the mixer to medium speed and beat for ~30 seconds. Gradually pour in the sugar syrup in a thin stream. Raise the mixer speed to med-high and whipe until the meringue has cooled to body temperature. This will take several minutes.

Gradually beat in the butter a few tablespoons at a time. Add the melted white chocolate and beat until the frosting is smooth and fluffy.

To Assemble the Cake
Put on layer, flat side up, onto a cake stand or serving plate.

Spread 3/4 cup of preserves over the cake, leaving a 1/4 inch margin around the edges. Top with the second layer. Cover with another 3/4 cup of preserves. Finally, put the third layer in place and frost the sides and top of the cake with the frosting.