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Preheat oven to 350 F; Grease and flour a 9" x 13" pan.
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To make the streusel topping, combine 1/4 cup flour, 2/3 cup sugar, and 1 teaspoon cinnamon.
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Rub in 1/4 cup butter until flour mixture resembles coarse breadcrumbs; set aside.
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In a large bowl, mix 2 cups flour, 3/4 cup sugar, baking powder and salt.
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Cut in 1/2 cup butter until mixture resembles breadcrumbs.
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Crack the egg into a measuring cup, and then add milk to make 1 cup.
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Pour into crumb mixture, and add the vanilla.
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Mix until it is just combined (Do not overbeat).
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Spread into pan, and sprinkle with streusel topping.
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Bake at 350 F for 25 to 30 minutes, or until done.
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Cool, and then cut into slices.