Spray a 10-inch fluted tube pan with nonstick cooking spray or grease and flour; set pan aside. In a medium bowl, stir together flour, baking powder, baking soda and salt; set aside.
In a large bowl, stir together eggs, 1 1/2 cup granulated sugar, butter and 1 1/2teaspoon vanilla. Add flour mixture to egg mixture; stir just until moistened. Add sour cream and stir vigorously until batter is smooth.
In a small bowl, stir together walnuts, brown sugar, 1/2 cup granulated sugar, cinnamon and 2 teaspoons vanilla.
Spoon one-third of the batter into the bottom of the prepared pan; spread evenly. Sprinkle with one-third of the streusel. Repeat layers with remaining batter and streusel.
Bake in a 325 degree F oven for 40 to 50 minutes or until a wooden toothpick inserted near the center comes out clean. Cool cake in pan on a wire rack for 10 minutes. To remove cake from pan, cover with an inverted serving plate and turn cake onto plate. Serve warm. Makes 12 servings.