Preheat oven to 350. Grease and flour a 12 cup bundt pan. Make streusel by combiniong all the ingredients and using a pastry cutter or two knives until mixture is crumbly. Make the glaze by combining powder sugar and milk or oj which ever you choose and mix until smooth.
In a medium bowl mix flour, cinnamon, baking soda and salt. In a large bowl, beat butter and sugar until light and fluffy. Add eggs one at a time, beating well after each one. Add pumpkin, sour cream and vanilla and mix well. Slowly add flour mixture.
To Assemble: Spoon half of the batter into prepared pan. Sprinkle streusel on top of batter in the center, make sure the streusel doesn't touch the sides. Top with remaining batter, make sure the batter on top reaches and touches the sides of the pan (this helps "lock" in the streusel).
Bake 55-60 mins. or until toothpick comes out clean. Cool for 30 mins. Invert onto wire rack and drizzle with glaze.