Making the Cake:

1. Position the rack in the center of the oven and preheat it to 350° F. Lightly butter a 10" tube pan. Dust the pan with flour. Tap out the excess.

2. Sift together the flour, 1/4 teaspoon of the salt and the baking soda. In a large bowl bowl, using a hand-held electric mixer set at high speed, beat the butter until creamy, about 1 minute. Add the sugar and beat until light in color and texture, about 2 minutes. Beat in the egg yolks one at a time, beating well after each addition. Then add the vanilla. Reduce the speed to low, and in thirds, alternately beat in the flour mixture and the sour cream.

3. In a large grease-proof bowl, using a hand-held electric mixer and with clean beaters, set at low speed, beat the egg whites until foamy. Now, add the remaining 1/4 teaspoon salt, increase the speed to high, and beat until the whites just form stiff peaks. Stir about one fourth of the beaten whites into the batter, then fold in the remaining white. Transfer the batter into the prepared cake pan, smoothing the top.

4. Bake until the cake is golden brown and a long broom straw or bamboo skewer inserted into it comes out clean, about 1 1/4 to 1 1/2 hours.

5. Cool the cake completely on a wire rack. Run a long think knife around the edges to release the cake from the pan, then unmold.