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1
Put oven rack in middle position and preheat oven to 375u00b0F.
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2
Pulse 1/2 cup nuts in a food processor until finely chopped, being careful not to process to a paste, then transfer to a large shallow baking pan. Toast nuts, stirring every 2 minutes to prevent edges from burning, until pale golden, 6 to 8 minutes. Cool completely in pan.
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3
Whisk together flour and cornstarch in a bowl.
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4
Beat together butter and sugar in a large bowl with an electric mixer at medium-high speed until pale and fluffy, about 3 minutes in a stand mixer or 5 with a handheld. Add toasted ground nuts and salt and beat until combined. Reduce speed to low, then add flour mixture and mix just until a dough forms.
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5
Form dough into a rectangular block and roll out between 2 (12-inch-long) sheets of parchment with a rolling pin into a 10- by 8-inch rectangle.
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6
Chill dough in parchment on a baking sheet until firm, about 10 minutes.
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7
Meanwhile, coarsely chop remaining 1/4 cup nuts.
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8
Discard top sheet of parchment and sprinkle dough with coarsely chopped nuts. Bake until golden, 14 to 16 minutes. Transfer shortbread to a rack and cool completely, then break into rough shapes with your hands.
