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1
Preheat the oven to 425u00b0 F. Line a baking sheet with parchment paper.
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2
To make the filling, squeeze the grapes into a large bowl, separating the flesh from the skin. Reserve the skins in a separate bowl. Transfer the pulp to a medium pot, and bring to a simmer over low heat. Simmer until the grapes have broken down to juice, about 10 minutes.
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3
Strain the grape mixture into the bowl with the skins-this is to remove the grape seeds, but really press the juice through the sieve to make sure you don't lose any. Cool the grape mixture completely.
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4
When the grape mixture is cool, whisk the sugar, cornstarch, and salt to combine. Stir this into the grape mixture, then stir in the vanilla.
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5
On a lightly floured surface, roll out half of the dough to 1/4 inch thick. Transfer the dough to the prepared baking sheet, then use a pastry wheel to cut it into a rectangle.
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6
On a lightly floured surface, roll out the second piece of dough to 1/4 inch thick. Use a pastry wheel to cut the dough into long strips. Use the first strip you cut as a guide for the rest so that they are even.
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7
Spoon the filling into an even layer in the center of the rectangle, leaving 2 inches of uncovered dough all around.
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8
Arrange the strips of dough at a diagonal across the filling, and use scissors to trim away the excess dough from the strips.
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9
Fold in the sides of the rectangle first, folding over the edges of the lattice strips and the filling. Then fold over the top and bottom of the rectangle to encase the strips and the filling. Be sure to apply a little pressure at the corners where the edges of dough meet-make sure they're sealed well or filling may leak out during baking!
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10
Egg wash the dough and sprinkle with turbinado sugar. Bake until the crust is golden brown and the filling is bubbly, 28 to 35 minutes.
