Chocolate Chip Coffee Cake
Nonstick vegetable oil spray
2 large egg whites (1/4 cup)
1/3 cup (packed) golden brown sugar
1-1/2 cups coarsely chopped walnuts
1-1/4 cups mini semisweet chocolate chips, divided
2 cups cake flour
1/2 teaspoon salt
1/4 teaspoon baking powder
1/4 teaspoon baking soda
1 cup plus 2 tablespoons sugar
1/2 cup (1 stick) unsalted butter, room temperature
3 large eggs
3/4 cup sour cream
3/4 cup powdered sugar
2 tablespoons sour cream
CAKE: Position rack in center of oven and preheat to 350ºF. Butter and flour 10-inch-diameter angel food cake pan with removable bottom, then spray with nonstick spray. Mix egg whites, brown sugar, and salt in medium bowl. Mix in walnuts and 1/4 cup chocolate chips.
Whisk flour, salt, baking powder, and baking soda in another medium bowl. Using electric mixer, beat sugar and butter in large bowl to blend. Beat in eggs 1 at a time. Stir in flour mixture in 4 additions alternately with sour cream in 3 additions. Stir in remaining chocolate chips.
Transfer batter to prepared pan; smooth top. Spoon walnut mixture evenly over. Bake cake until tester inserted near center comes out clean, about 1 hour 8 minutes. Cool cake in pan on rack 15 minutes. Using sharp knife, cut around sides of pan and center tube to loosen cake. Using oven mitt, grasp center tube and lift from sides of pan. Cool cake completely on rack. Cut cake free from pan bottom. Placing hand gently atop cake for support, tilt cake just far enough over to release from center tube; remove tube. Place cake, streusel side up, on platter (walnuts should be on top).
DO AHEAD: Can be made 1 day ahead. Wrap in plastic.
TOPPING: Mix sugar and sour cream in bowl; drizzle over cake.