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1
Mix the cornstarch and powdered sugar together (with the yogurt powder if using).
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2
Oil an 8x8-inch pan and coat it with some of the cornstarch mixture. Tap out the excess and add it to the remaining cornstarch mixture.
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3
Put the egg white, 1/4 cup of water, and vanilla a mixer bowl. Sprinkle gelatin evenly over the surface; do not mix it in. Let set for 5 minutes to soften; the gelatin should look opaque but no longer dry. Set the bowl in a pan of very hot tap water until the gelatin is melted-the mixture should look clear and no longer opaque.
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4
Meanwhile pour the remaining 1/2 cup of water, corn syrup, and salt in a small (1 quart) sauce pan. Pour the sugar in the center of the pan, forming an island that does not touch the sides of the pan. Don't stir; just pat the island to moisten any dry sugar. Cover the pan and bring the mixture to a simmer over medium heat. Uncover and peek from time to time over the next 5 minutes. When the sugar looks dissolved, continue to cook uncovered, always without stirring, to 260u00b0F.
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5
Meanwhile, put the bowl with the melted gelatin onto the mixer and attach the whisk.
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6
As soon as the syrup is at temperature, turn the mixer to high and start pouring the syrup gradually down the inside of the bowl in a thin continuous stream. Continue to beat at high speed for 8 to 10 minutes, until the mixture is very thick and fluffy; it should change from white and shiny to white with a slightly dull gloss; the mixer bowl should be barely lukewarm.
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7
Turn the mixer of and set the whisk aside. Pour the yogurt powder into the bowl. Use the whisk attachment (or a spatula) to mix/fold the powder partially into the marshmallow-just a few strokes to prevent the powder from flying around later. Put the bowl back on the mixer and attach the whisk. Beat for a few seconds to mix the powder thoroughly into the marshmallow. Scrape the marshmallow into the prepared pan and spread it even. Let set for at least 3 hours.
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8
Dust a cutting board liberally with some of the remaining starch and sugar mixture. Slide a thin spatula around the sides of the pan to detach the marshmallow. Tip the pan over the cutting board and pry the marshmallow gently from the bottom of the pan onto the cutting board. Sieve more starch and sugar over the top of the marshmallow. Cut the marshmallow into cubes with knife and toss with the remaining starch/sugar-this is easiest to do by pouring the cubes and starch mixture back and forth between two bowls. If necessary, shake the marshmallows gently in a coarse mesh sieve to remove excess starch. Let dry for a few more hours at room temperature before storing in a sealed container.
