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1
Prepare a 12 x 17 inch cookie sheet by covering it with aluminum foil or parchment paper and spraying it with cooking spray. Fit a standing kitchen mixer with a whisk attachment.
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2
Place 1/3 cup cold water in the bowl of an electric mixer and sprinkle the gelatin on top. Let it soften while you complete the next steps.
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3
Place the corn syrup, sugar, salt, and 1/3 cup water in a medium saucepan over medium-high heat. Stir until sugar dissolves and then continue to boil, without stirring, until mixture reaches 238 degrees (soft-ball stage) on a candy thermometer.
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4
Carefully remove the sugar saucepan from the heat. Turn mixer to low speed and, while mixer is whisking gelatin, slowly and carefully pour the hot syrup down the sides into the bowl.
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5
Once all syrup has been added, increase the mixer speed gradually until it is mixing on high. Beat until mixture is thick, white and is three times the original volume (about 10 minutes). Add cinnamon oil and food coloring and mix until combined.
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6
Using a well-oiled spatula, turn the marshmallow out of the mixer bowl onto the cookie sheet and spread in a thin layer evenly over the whole sheet. Let stand at room temperature until completely set, 5 hours (or overnight).
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7
Spray a heart-shaped cookie cutter with cooking spray, and stamp out heart marshmallows. Store in an airtight container for up to one week.
