Preheat oven to 350°.
Combine the yolks, about ¼ of the sour cream, and the vanilla in a medium mixing bowl.
Combine the dry ingredients in a large mixing bowl, and mix on low for about 30 seconds. Add the butter and remaining sour cream to this mixture. Mix on low until the dry ingredients are moistened.
Increase to medium speed (high if using a hand mixer) and beat for about 1½ minutes. Be sure to scrape down the sides.
Gradually add in the egg mixture in three batches, beating for 20 seconds after each addition. Scrape down the sides.
Reserve about ⅓ of the batter and scrape the remainder into the prepared pan (see equipment list).
Smooth the surface and sprinkle with the streusel filing. Top with apple or peach slices if desired.
Drop the reserved batter in large blobs over the fruit and spread evenly with the spatula. Sprinkle with the streusel topping and bake for 55 - 65 minutes, or until a cake tester or toothpick comes out clean when pressed into the center. Cover loosely with buttered foil after 45 minutes to prevent over browning.
Let the cake cool in the pan on a rack for 10 minutes. Loosen the sides and remove the sides of the springform pan. Cool completely before wrapping airtight.