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1
Preheat oven to 350u00b0 F with a rack in the lowest position. Spray a Bundt pan with cooking spray that contains flour or brush with a mixture made from 1 tablespoon melted butter and 1 tablespoon flour. (A great trick I learned from Cook's Illustrated for Bundt cakes.)
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2
Combine streusel ingredients in the bowl of a food processor. Process to combine. Transfer to a bowl and set aside. (If your brown sugar is free of clumps, you could also just whisk the mixture together.)
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3
Place sugar for cake in processor bowl, then process 60 to 90 seconds to create a finer sugar, which helps to create a finer texture in the final cake. Transfer the sugar to the bowl of a stand mixure.
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4
Wash the oranges, then cut off and discard the stem and blossom ends. Cut the oranges into about 1-inch chunks (removing any seeds), and place them in the food processor. Process in 3 to 4 second pulses until the mixture is fairly smooth, but not completely pureed. There should be visible pieces of peel left, but they should be fairly small (pinky nail-sized). Measure out 1 1/2 cups (~365 g) in a 2- to 4-cup glass measure. Add the eggs and 1 tablespoon vanilla to the glass measure, and whisk with a fork to combine (being careful if using the 2-cup measure, as there won't be much room for splashing).
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5
Transfer the remaining orange mixture to a fine-mesh strainer and press out as much juice as possible. Set the juice aside for making the icing. Discard the pulp.
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6
Add the flour, baking soda, baking powder, and salt to the mixing bowl with the sugar. Mix on low for 30 seconds to combine. Add the butter and half of the orange mixture. Mix on low until the flour is mostly moistened, then increase the speed to medium and mix for 30 seconds. Scrape down the sides of the bowl. Add half of the remaining orange mixture, and mix on medium speed for 20 seconds. Add the remaining orange mixture, and mix on medium speed for 20 seconds. Scrape the bowl, then mix on medium-high for 60 seconds. Scrape the sides of the bowl.
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7
Transfer about a quarter of the batter to the prepared Bundt pan, and smooth into an even layer using a rubber spatula. Sprinkle evenly with 3/4 cup of the streusel. Transfer half of the remaining batter to the pan, and smooth into an even layer. Sprinkle with remaining streusel. Transfer and smooth the remaining batter.
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8
Bake for 45 to 55 minutes, until a bamboo skewer inserted in the center comes out clean or with only streusel crumbs attached.
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9
While the cake is baking make the icing: Whisk together the powdered sugar, strained orange juice, and 1/2 teaspoon vanilla. Taste and add lemon juice if desired. Because the amount of orange juice will vary, adjust the icing until it is a fairly thick drizzling consistency. Add orange juice, milk, half-and-half, or powdered sugar by the tablespoon as needed. Cover bowl tightly with plastic wrap, and set aside.
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10
Transfer pan to a wire rack and allow to cool for 25 to 30 minutes. Invert the cake onto the pan and allow the cake to cool an additional 30 minutes or more before drizzling the icing on (the cake should be barely warm to cool before icing). I like to drizzle the icing while the cake is on the wire rack set over the sink, so clean up is a breeze. Allow the cake to fully cool and the icing to set before serving. Keeps tightly wrapped for several days.
