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1
Preheat oven to 350 F with a rack in the bottom third of the oven. Spray a 13x9 pan with cooking spray, then line the pan with aluminum foil or parchment (leaving an overhang so you can lift the bars out) and spray again with cooking spray.
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2
To make the dough: In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, sugar and salt on medium speed until soft and light, about 2-3 minutes. Beat in the vanilla.
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3
With the mixer on low, add 2 1/4 cups of the flour, beating just until the dough is smooth and the flour has been absorbed, and stopping to scrape the sides of the bowl and the paddle as necessary.
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4
Transfer 3/4 of the dough into the prepared baking pan. Press the dough evenly into the pan to form the bottom layer. Transfer the pan to the refrigerator while you make the filling and top layer.
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5
Add the remaining 1/4 cup of flour to the remaining dough in the mixer bowl and work it in with your fingertips until 1/8 - 1/4-inch crumbs form. Set aside.
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6
To make the filling: In a medium saucepan, combine the butter, corn syrup, brown sugar and condensed milk. Bring to a simmer, whisking occasionally. When the mixture begins to boil, whisk frequently until it begins to thicken and darkens in color slightly, about 10 minutes. Remove from the heat and let cool for about 10 minutes.
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7
Remove the pan from the refrigerator and pour the filling over the bottom crust. Use an offset spatula to spread it evenly then distribute the crumb topping over the filling.
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8
Bake for about 30 minutes, or until the filling is gently bubbling and a dark caramel color. Cool the bars in the pan on a wire rack until lukewarm, about 15-20 minutes. Use the foil (or parchment) to lift the bars out of the pan and onto a cutting board. Once completely cool, cut into 2-inch squares.
