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1
For the blueberry swirl:
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Add sugar, blueberries, lemon juice, lemon zest, and salt to a small saucepan and heat over medium heat, smashing some of the berries against the side or bottom of the pan with a fork or pastry cutter. Bring the mixture to a boil and continue to cook over medium heat, stirring frequently, for 10-15 minutes, until mixture is thickened. If you drip some onto a plate, the blueberry sauce should hold its shape.
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3
Remove pan from heat and stir in vanilla. Scoop into a bowl and allow to cool completely in the refrigerator before using in the blueberry cake. (The blueberry sauce will keep in the refrigerator for up to 4 days.)
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4
For the crumb topping:
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5
Add sugar, brown sugar, cinnamon, salt, and melted butter to a medium size bowl and whisk vigorously to combine.
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6
Whisk flour and cornstarch and add to the butter and sugar mixture. Using your hands or a rubber spatula, rub the flour into the butter until completely moistened. Cover and set aside.
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7
For the cake:
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Preheat oven to 325u00b0F. Grease a 9- or 10-inch springform pan and cover bottom of pan with parchment paper. Shake some flour around inside the greased pan to coat completely.
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Add flour, cornstarch, sugar, baking soda, and salt to the bowl of an electric mixer fitted with the paddle attachment. Mix on low just to combine. With the mixer running on medium-low speed, add butter, piece by piece, mixing until butter is completely incorporated into flour and mixture looks like coarse crumbs. Add eggs and egg yolk, buttermilk. and vanilla and beat on medium speed for 1 minute to combine.
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10
Scoop batter into prepared springform pan and smooth into an even layer. Drop spoonfuls of blueberry sauce all over the top of the cake batter. With a butter knife, swirl the blueberry sauce into the cake batter. Sprinkle the crumb topping over the top of the cake, squeezing it in your hands as you do to create large pieces of crumb.
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11
Bake the blueberry crumb cake for 60-70 minutes. The cake is done when a toothpick inserted in the center comes out without any raw batter on it.
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12
Remove the cake from the oven and allow to cool on a wire rack for at least 15 minutes before removing the sides of the springform pan. Serve warm or at room temperature. The cake will keep in an airtight container at room temperature for up to 2 days.
