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1
Invert a baking sheet and spray it with nonstick cooking spray. Line the top with parchment paper. With an offset spatula, spread the melted chocolate evenly across the parchment. Place the baking sheet in the freezer until firm, approximately 30 minutes.
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2
Lightly coat a quarter sheet (9 by 13-inch) pan or glass baking dish with nonstick cooking spray and line with parchment paper, leaving 1 inch of overhang on the long sides.
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3
In the bowl of a stand mixer fitted with the paddle attachment, mix the butter on medium speed for 5 to 10 seconds. Add the granulated sugar and beat the butter mixture until it is aerated, 3 to 4 minutes. Scrape the sides and bottom of the bowl with a rubber spatula to bring the batter together.
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4
Add the water and vanilla to the egg yolks. On medium speed, add the yolks, one at a time, mixing briefly until the batter resembles cottage cheese, approximately 5 seconds per yolk. Scrape the sides and bottom of the bowl with a rubber spatula to bring the batter together. Mix on medium speed for 20 to 30 seconds to make it nearly homogeneous.
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5
In a bowl, whisk together the flour, baking powder, baking soda, and salt.
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6
Add the flour mixture all at once and mix on low speed until the dough just comes together but still looks shaggy, approximately 30 seconds. Do not overmix. Remove the bowl from the stand mixer. With a plastic bench scraper, bring the dough completely together by hand.
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7
Transfer the dough to the prepared pan. Using a rubber spatula and then your fingertips, press the dough into the corners of the pan and smooth the surface.
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8
Remove the chocolate from the freezer and break it up into shards. Scatter the chocolate shards across the surface of the dough and then press into the dough. Cover the top with plastic wrap, pressing down through the plastic to smooth down the top. Refrigerate until the dough is set, at least 20 minutes or overnight. (If refrigerating the dough overnight, refrigerate the egg whites as well, letting them come to room temperature before proceeding with the meringue.)
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9
Preheat the oven to 350u00b0F.
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10
In a stand mixer fitted with the whisk attachment, whip the egg whites and a pinch of kosher salt on medium speed until frothy, approximately 45 seconds. Add the brown sugar and whip on medium-high speed until shiny peaks form and the meringue holds its shape, 2 to 3 minutes. Spread the meringue evenly over the chilled dough.
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11
Bake, rotating the pan halfway through the baking process, until the meringue resembles a lightly toasted marshmallow, 23 to 25 minutes. It will start to crack on the sides but should still be slightly soft in the center, and the shortbread should be cooked through but still pale-not golden brown. (Do not overbake or the shortbread will be too crumbly to cut.) Cool completely in the pan. Once cool, refrigerate until chilled.
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12
Lift the bars out of the pan using the parchment handles and transfer to a cutting board. Cut the bars lengthwise into 3 strips. Cut the bars crosswise into 10 strips to make small, rectangular bars. Serve the bars at room temperature.
