To the remainder add the flour, butter, and vanilla; mix well until it forms a coarse, crumbly mixture for the streusel topping.
Preheat oven to 350°F.
Prepare a 9-inch springform pan, greased, bottom lined with parchment or wax paper, and then greased again and floured; set aside.
In a medium bowl, lightly combine the yolks, about 1/4 of the sour cream, and vanilla.
In a large mixing bowl, combine the dry ingredients and mix on low speed for 30 seconds to blend.
Add the butter and remaining sour cream.
Mix on low speed until the dry ingredients are moistened.
Increase to medium speed (high speed if using a hand mixer) and beat for 1 1/2 minutes to aerate and develop the cake's structure.
Scrape down the sides.
Gradually add the egg mixture in 3 batches, beating for 20 seconds after each addition to incorporate the ingredients and strengthen the structure.
Scrape down the sides.
Reserve about 1/3 of the batter and scrape the remainder into the prepared 9-inch springform pan.
Smooth the surface, preferably with a small angled spatula.
Sprinkle with the streusel filling*.
Drop the reserved batter in large blobs over the streusel filling and spread evenly with a spatula.
Sprinkle with streusel topping.
Bake 55 to 65 minutes or until a wire cake tester inserted in the center comes out clean and the cake springs back when pressed lightly in the center.
(Move aside a small patch of streusel before testing.) The cake should start to shrink from the sides of the pan only after removal from the oven.
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.
The cake will have a level top.
Loosen the sides with a small metal spatula and remove the sides of the springform pan.
Cool completely before wrapping airtight.
If you wish to remove the bottom of the pan, slide a cardboard round at least 9 inches in diameter between the parchment and the bottom when the cake is completely cool.
*Optional: 1 heaping cup of sliced 1/4-inch thick Granny Smith Apples sprinkled with 2 teaspoons lemon juice OR 1 heaping cup of frozen peaches, thawed on paper towels and slice 1/4 inch thick while still partially frozen.
Either of these fruits may be placed on top of the streusel filling prior to dropping the reserved batter in large blobs.