Blend together 1/2 cup butter, 3/4 cup sugar and 3 eggs until creamy, then add the vanilla. Mix in the flour, baking powder and baking soda. Add the sour cream, then set batter aside.
Combine pumpkin, remaining egg and remaining sugar; stir in pumpkin pie spice, and set aside.
Prepare streusel by combining brown sugar, remaining butter, cinnamon and nuts; set aside.
Spray a 13" x 9" baking dish with non-stick spray. Spoon in half the batter; top with half the streusel, then all the pumpkin mixture. Spread on remaining batter, then top with remaining streusel mixture.
Bake at 325 degrees for 50 to 60 minutes, or until a toothpick inserted in the center comes out clean.