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1
Preheat the oven to 325u00b0F. Line the bottom of a 9-inch tube pan with parchment paper and butter and flour the pan.
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2
Sift the flour, baking soda, salt, and ground cardamom together. Set aside.
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3
Split the vanilla beans and scrape the contents into the sour cream. Stir until blended. Set aside.
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4
Beat the butter and 2 cups of sugar together in a large mixing bowl until lightened in color and texture, about 3 to 5 minutes.
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5
One at a time, beat in the egg yolks.
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6
Add the flour mixture in 3 parts, alternating with the sour cream mixture in 2 parts, beating on the lowest speed of a stand or a hand mixer or stirring with a wooden spoon. Scrape the sides of the bowl to ensure uniformity.
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7
In a separate mixing bowl, beat the egg whites and a pinch of salt. Begin on low speed. When the mixture begins to foam, sprinkle in the cream of tartar and gradually increase the speed to medium. When soft peaks form, gradually add the remaining sugar and continue beating until the peaks are stiff but still moist.
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8
Gently stir in 1/4 of the egg whites into the batter to lighten it, then fold in the remaining whites until evenly incorporated. Spread the batter into the prepared pan and bake until a toothpick inserted in the middle of the cake comes out clean, 1 hour 10 minutes to 1 hour 20 minutes. Let the cake cool in the pan on a cooling rack for 10 minutes. Slide a knife around the edge of the pan to loosen it and then invert the cake and cool it further right side up on the rack.
