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1
Place 1/2 cup water in small bowl. Sprinkle gelatin over; let stand until gelatin softens, about 10 minutes.
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2
Line 8x8x2-inch glass baking dish with plastic wrap, leaving overhang. Sprinkle 2 tablespoons powdered sugar over plastic wrap. Combine remaining 3/4 cup water, 2 cups sugar, and 6 tablespoons corn syrup in heavy small saucepan. Stir over medium heat until sugar dissolves. Boil without stirring until thermometer registers 230u00b0F, occasionally swirling pan and brushing down sides with wet pastry brush, about 14 minutes (time will vary depending on size of pan). Stir in remaining 6 tablespoons corn syrup and vanilla.
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3
Transfer sugar mixture to heavy-duty stand mixer with paddle attachment. Add gelatin mixture. Beat on low speed until mixture turns opaque, about 5 minutes. Beat on medium-high until mixture is cool and forms soft white peaks, about 15 minutes. Spread evenly in prepared dish. Dust with 2 tablespoons powdered sugar. Let stand at room temperature at least 8 hours to set (marshmallow will be soft). Cover; refrigerate. (Can be made 2 days ahead.)
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4
Using electric mixer, beat butter in medium bowl until smooth. Add sugar; beat until blended. Add flour, cocoa powder, and salt; beat just until blended, adding water by teaspoonfuls if mixture is too dry. Form dough into ball; flatten into rectangle. Wrap in plastic and chill at least 1 hour.
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5
Preheat oven to 325u00b0F. Line large rimmed baking sheet with parchment paper. Roll dough out on lightly floured surface to 1/4-inch-thick rectangle. Cut dough into 2x4-inch rectangles. Transfer rectangles to prepared sheet. Bake until puffed and beginning to crack, about 9 minutes. Transfer cookies to rack to cool (cookies will become crisp as they cool).
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6
Line another large rimmed baking sheet with parchment paper. Stir chocolate in double boiler over barely simmering water until melted and smooth. Spread over parchment on prepared sheet. Run fork up and down length of chocolate, forming ridges. Refrigerate until firm, about 30 minutes. Break chocolate into large irregular pieces.
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7
Place 1 cookie on each of 6 plates. Cut marshmallow into six 2x4-inch rectangles. Transfer marshmallows to sheet of foil. Using blowtorch, brown tops of marshmallows. Invert 1 marshmallow, brown side down, atop each cookie. Brown top and sides of marshmallows on cookies. Arrange chocolate pieces decoratively atop marshmallows and serve.
