Pre-heat oven to 350F. Grease and flour 12-cup Bundt pan. Bake for 1 hr and then check cake to see if done. If not, continue to cook an additional 5 minutes.
Combine flour, cinnamon, baking soda and salt in medium bowl. Beat sugar and butter in large mixer bowl until light and fluffy. Add eggs one at atime, beating well after each addition. Add pumpkin, sour cream and vanilla extract. Gradually beat in flour mixture.
Streusel topping: Combine all ingredients and cut in the butter with 2 knives making a crumbly mixture.
To assemble spoon 1/2 the batter into prepared pan. Sprinkle the streusel mixture on top and then remainder of batter on top.
Dust with Powdered Sugar or serve with Whipped Topping.