A rich “plain cake” with a nuance of sour cream and a spectacular crackly brown top. Leftovers stay moist for close to a week.
Have all ingredients at room temperature, 68° to 70°F. Preheat the oven to 325°F. Grease and flour one 9-inch tube pan.
Sift together:
      3 cups sifted cake flour
      1/4 teaspoon baking soda
      1/4 teaspoon salt
Combine:
      1 cup sour cream
      2 teaspoons vanilla
In a large bowl, beat until creamy, about 30 seconds:
      1/2 pound unsalted butter
Gradually add and beat on high speed until lightened in color and texture, 3 to 5 minutes:
      2 cups sugar
Beat in 1 at a time:
      6 large egg yolks
Add the flour mixture in 3 parts, alternating with the sour cream mixture in 2 parts, beating on low speed or stirring with a rubber spatula until smooth and scraping the sides of the bowl as necessary. In another large bowl, beat on medium speed until soft peaks form:
      6 large egg whites
      1/4 teaspoon cream of tartar
Gradually add, beating on high speed:
      1/2 cup sugar
Beat until the peaks are stiff but not dry. Use a rubber spatula to fold one-quarter of the egg whites into the sour cream mixture, then fold in the remaining whites. Scrape the batter into the pan and spread evenly. Bake until a toothpick inserted into the center comes out clean, 1 hour 10 minutes to 1 hour 20 minutes. Let cool in the pan on a rack for 10 minutes. Slide a thin knife around the cake to detach it from the pan. Invert the cake, then let cool right side up on the rack.