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1
Sift together potato starch and confectioners sugar.
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2
Prepare a 13- by 9-inch baking pan by lightly buttering inner surface and sifting sugar-starch powder over it.
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3
Pour 1/2 cup of water into the bowl of a stand mixer fitted with a whisk attachment. Sprinkle gelatin powder into water and let stand.
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4
Pour remaining 1/2 cup of water and sugar into a saucepan over medium-high heat. Stir occasionally until sugar is dissolved. Leave undisturbed once sugar is dissolved. When sugar is dissolved and syrup begins to boil, leave undisturbed until temperature on a candy thermometer reaches 240u00b0F, about 5 minutes.
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5
Carefully pour the hot syrup into the bowl, gently stirring until gelatin is fully dissolved in the syrup. Gradually increasing the speed to high, beat the mixture until almost tripled in volume and thick, shiny, soft peaks have formed, about 10 minutes. Add vanilla extract.
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6
While syrup/gelatin mixture is beating, pour egg whites and pinch of salt in a deep mixing bowl. With an electric hand mixer, whisk eggs until they begin to foam. Sprinkle in cream of tartar, gradually increase speed to high and beat until shiny and firm peaks form, about 3 minutes.
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7
Switch to paddle attachment on the stand mixer and incorporate egg whites into syrup/gelatin mixture on medium-low speed, stopping to scrape the bottom and sides of the bowl. When mixture is fully blended, after 2-3 minutes, spread into prepared pan.
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8
Sift sugar-starch powder over the top surface of the marshmallow. Allow to set overnight. If the air is very dry, cover partly with a plate or plastic wrap.
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9
Run a small offset spatula around edges of pan and turn marshmallow out onto a surface that has been dusted with sugar-starch powder.
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10
Use cookie cutters, scissors, or a sharp chef's knife to cut the marshmallow into pieces. If using a knife, cut into cubes by pressing down, not sawing back and forth - it will bounce back. Dip cut marshmallows into sugar-starch powder to coat evenly, shaking off excess powder. Store up to a week in a closed container.
