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1
Spray an 8x8, 9x13 or similar baking pan with the cooking spray or generously grease using vegetable oil. Lay a sheet of parchment paper inside the pan, making sure all inner sides of the pan are covered. Depending on the size of your baking pan, you may need to use two sheets, criss-crossed. You should have 1-2 inches overhanging the pan, as this will be how you pull the marshmallows out of the pan once they have set.
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2
Pour 1/2 cup of the water into the bowl of a stand mixer fitted with a whisk attachment. Add the gelatin to the water and allow to soften, about 10 minutes.
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3
In a large, deep saucepan, combine sugar, evaporated milk, and remaining 1/4 cup water. Stir over medium-heat until the sugar has dissolved. Bring the water/sugar mixture to a gentle boil. Reduce to a simmer, without stirring, and allow to cook until a candy thermometer registers 240u00b0F, about 10-15 minutes. Make sure the heat is not too high or the mixture will burn while cooking.
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4
Carefully, add hot sugar mixture to the gelatin mixture and beat on low speed until fully incorporated. You'll note the mixture is an amber color-this is totally normal. Once it starts to get whipped, it will turn into a creamy, shiny, white-ish color.
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5
Add vanilla extract. Increase speed to high and beat until very thick and fluffy, about 10-15 minutes. The mixture will double to triple in size and will be a creamy, shiny, white-ish color. Using a rubber spatula, quickly scrape out the mixture into the baking pan lined with parchment paper. The mixture will set quickly, so try to do this as fast as possible. It will be difficult to get every last drop of the marshmallow mixture out of the mixing bowl, so just get what you can. Then, using the spatula, press the mixture down into all the corners of the pan. Finally, smooth out the top as best as you can.
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6
Using a final sheet of parchment paper, lay over the top of the mixture. Using your hands, gently press this sheet down over the top so the mixture sticks to it. This will prevent all sides of the marshmallows from getting dried out while they are setting. Allow to set (not refrigerated) for at least 8 hours, preferably overnight.
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7
After the marshmallows have set, combine the powdered sugar and cornstarch in a large bowl and whisk together. Spoon some of the mixture over the top of the marshmallows and over a cutting board. Using the overhanging parchment paper, remove the marshmallows from the pan and place onto your board. Carefully peel the paper off the marshmallows.
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8
Using a large, sharp knife, cut the marshmallows into 1-inch squares. The very edges may have dried out too much from setting overnight, so just cut them off and discard as necessary. Add the marshmallows to the powdered sugar and cornstarch mixture and dust all sides to prevent sticking.
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9
Store in an airtight container for several weeks.
