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1
Preheat oven to 350 F. Spray a 9-inch springform pan with nonstick cooking spray with flour. (A 9-inch cake pan works too as long as it has about 3-inch sides.)
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2
To make the streusel: Stir the flour, oats, brown sugar, cinnamon and salt together in a medium bowl. Add the butter, and use a pastry cutter to work it into dry ingredients until the mixture resembles coarse crumbs. Set aside while you make the batter.
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3
To make the batter: Whisk the flour, baking powder, baking soda, cinnamon, ginger, nutmeg and salt together in a medium bowl. In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and brown sugar on medium speed until light and fluffy, about 3 minutes. Add the eggs, one at a time, beating well after each addition, and scraping down the sides of the bowl as necessary. Add the pumpkin puree and sour cream and beat until incorporated. With the mixer on low speed, add the dry ingredients, beating just until combined - the batter will be very thick.
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4
Spread half of the batter in an even layer in the bottom of the prepared pan. Sprinkle half of the streusel mixture over the batter. Dollop the remaining batter over the streusel and use an offset spatula to spread it as best you can (it doesn't have to be perfect). Top with the remaining streusel.
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5
Bake the cake for about 45-50 minutes, or until a toothpick inserted in the center comes out clean. Transfer the pan to a wire rack and let cool for 15 minutes then remove the sides of the pan and let the cake cool completely.
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6
To make the glaze: Whisk the confectioners' sugar, milk and vanilla in a small bowl until the consistency is thick, but pourable. You might need to add a little more milk to achieve the right consistency (I did). Drizzle over the top of the cake. Let the glaze set before serving.
