Preheat oven to 350°.
To prepare streusel, combine first 3 ingredients in a small bowl; set aside.
To prepare cake, beat 2 cups sugar, butter, and cream cheese at medium speed of a mixer until well-blended (about 5 minutes). Add egg whites and egg, 1 at a time, beating well after each addition. Lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 3 ingredients (flour through salt), stirring well with a whisk. Add flour mixture to sugar mixture alternately with sour cream, beginning and ending with flour mixture. Stir in vanilla.
Spoon half of batter into bottom of a 12-cup Bundt pan coated with cooking spray; sprinkle with streusel. Spoon remaining batter over streusel; spread evenly. Bake at 350° for 1 hour or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack; remove from pan. Cool completely on wire rack. Sprinkle with powdered sugar.
Note: We also tested this cake substituting 1 cup light butter (such as Land O Lakes) for 10 tablespoons regular butter. Because the total fat was slightly lower using light butter, we discovered we could increase the walnuts from 1/4 cup to 1/2 cup and still have about the same amount of calories and fat.