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To make the filling: In a medium pot, combine the blueberries, maple syrup, and salt. Cook over medium-low heat, stirring occasionally, until the blueberries begin to burst and release their juices, about 4 to 6 minutes.
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Lightly mash some of the blueberries with a flexible spatula or wire masher. Increase the heat and bring the mixture to a boil.
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In a small bowl, stir together the lemon juice and cornstarch until smooth; this is known as a “slurry.” Stir the cornstarch slurry into the blueberry mixture and boil for 30 seconds (and no longer), then remove from the heat. The mixture will visibly thicken.
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Stir in the vanilla and set aside to cool while you prepare the crust and topping.
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To make the crust and topping: Preheat the oven to 375°F and prepare an 8" square pan with a parchment sling, then lightly grease the sides of the pan.
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In a large bowl or the bowl of a stand mixer, combine the flours, oats, sugars, salt, cardamom, baking powder, and baking soda. Add the butter and vanilla and mix at low speed using an electric hand mixer or the flat beater attachment of a stand mixer until the mixture is crumbly. The butter should be about the size of peas and the mixture should hold together when squeezed.
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Transfer half of the crumbs (about 2 cups, or 260g) to the prepared pan. Shake the pan to evenly distribute the crumbs, then use your hands or the back of a flat measuring cup to press firmly into the edges and corners of the pan. Refrigerate the remaining crumbs until ready to use as the topping.
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Bake the crust for 15 to 20 minutes, or until well browned. Remove from the oven and cool for 10 minutes.
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Transfer the blueberry filling to the pan and gently spread evenly over the crust. (A small offset spatula is a helpful tool here.) Remove the remaining crumbs from the refrigerator and sprinkle them evenly over the top of the blueberry filling.
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Return the blueberry bars to the oven and bake for 25 to 30 minutes, until the crumbs are brown and the filling is bubbling around the edges.
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Remove the blueberry bars from the oven and allow them to cool completely in the pan before using the parchment to transfer them to a wire rack. Slice and serve at room temperature.
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Storage information: Blueberry bars are best enjoyed the same day they're baked. Leftover blueberry bars can be stored, covered, at room temperature for several days, but they'll soften over time.
