Streusel Kuchen

Streusel:
1 cup all purpose flour
2/3 cup (packed) golden brown sugar
1/3 cup chopped walnuts
1/4 teaspoon salt
6 tablespoons (3/4 stick) unsalted butter, melted

Cake:
2 medium-size Golden Delicious apples, peeled, halved, cored,
cut into 1/8-inch-thick slices
1/2 teaspoon ground cinnamon
1/8 teaspoon ground cloves
1 1/2 cups plus 2 tablespoons cake flour
1/3 cup cornstarch
1 1/2 teaspoons baking powder
1/4 teaspoon salt
1 cup plus 2 tablespoons unsalted butter (2 1/4 sticks),
room temperature
2 cups powdered sugar
1 tablespoon finely grated lemon peel
1 vanilla bean, split lengthwise
3 large eggs plus 1 large egg yolk

For streusel: Mix first 4 ingredients in medium bowl. Add melted
butter and stir with fork until moist clumps form. Set aside.

For cake: Position rack in center of oven and preheat to 325F.
Butter and flour 10-inch-diameter springform pan. Toss apple slices,
cinnamon, and cloves in medium bowl. Sift flour, cornstarch, baking
powder, and salt into another bowl. Combine butter, powdered sugar,
and lemon peel in large bowl; scrape in seeds from vanilla bean.

Using electric mixer, beat butter mixture until well blended. Add
eggs 1 at a time, beating until well blended after each addition;
beat in egg yolk. Add flour mixture and beat just until blended.
Spread batter evenly in prepared pan (cake layer will be thin).
Slightly overlap apple slices atop batter in concentric circles.
Sprinkle streusel evenly over apples (streusel will cover apples).

Bake cake until streusel topping is crisp and tester inserted into
center of cake comes out clean, about 1 hour 20 minutes. Cool cake
in pan on rack 15 minutes. Remove pan sides; cool to lukewarm.
Serve lukewarm or at room temperature.