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1
Line a baking sheet with parchment or wax paper. Beat the egg with a splash of water in a small bowl; set it and a pastry brush on the counter.
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2
Dust a work surface with flour. Dust the puff pastry dough with flour, then roll it into a 16-inch square. Flip the dough over as you go, flouring it as needed, so you roll both sides; if it gets soft and sticky, chill the dough for a few minutes, then carry on. If you end up with a rectangle, turn the dough so that a short side faces you. Brush the flour off the dough.
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3
Brush the dough's surface with the egg wash, then sprinkle with sugar, coating but not burying.
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4
Fold it in half (the long way if you have a rectangle) to create an approximately 8-by-16-inch rectangle. Once again, brush off the flour, brush on the egg wash and sprinkle with sugar.
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5
Fold the puff pastry again lengthwise to create an approximately 4-by-16 rectangle. Brush off the flour, brush on the egg wash and sprinkle with sugar. Slide the piece of folded pastry onto the lined baking sheet; freeze for at least 2 hours.
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6
Preheat the oven to 400u00b0 F. Line two baking sheets with parchment paper or a Silicone mat.
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7
Remove the puff pastry from the freezer, leaving it on the paper, and slide it onto a cutting board. (Keep a long side facing you.) Use a ruler and a pizza wheel or a sharp knife to cut the dough crosswise into 1/4-inch-wide strips.
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8
Place one strip of pastry sheet on a lined baking sheet, cut side up, so the layers are visible. Line up 2 more cut-side-up strips next to it and gently press them together (you'll have groups of 3). You don't want to squash them and pinch their layers, but you do want to get them so that they're touching one another and will bake together as one cookie. Repeat until you've used all of the strips (to form 16 cookies).
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9
Sprinkle the tops lightly with sugar. (Divide the cookies between the two baking sheets; keep one in the refrigerator while the other one bakes.)
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10
Cover the pastries with a sheet of parchment paper and top with an inverted ovenproof cooling rack. The parchment paper and rack will help keep the pastries flat and crisp. Bake one sheet at a time for 10 minutes, then remove the cooling rack and the top sheet of parchment.
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11
Press the pastries down with a metal spatula and flip them over. Bake uncovered for 7 to 10 minutes or until golden. Transfer to a rack to cool to room temperature.
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12
Just before serving, sandwich pairs of the pastries together with jam. If desired, use a piece of parchment paper to shield all but a 1/2-inch margin at the two ridged ends while you dust those ends with confectioners' sugar. (That way, you still get to see the layers in the pastry.)
