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1
Grease an 8- by 8-inch pan lightly with nonstick spray. Line the pan with parchment paper (I cut the paper at the corners so that they fit flush to the pan). Butter the top of the parchment with 1 tablespoon of butter. Butter the inside of an electric mixer bow with another tablespoon of butter.
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2
In a medium pot, combine the sugar, brown sugar, corn syrup, salt, milk, and peanut butter (be sure to use a pot that is large enough to allow the mixture to vigorously bubble without bubbling over). Clip a candy thermometer to the side of the pot. Cook over medium-low heat, stirring constantly to combine the ingredients and help the sugar dissolve. Once the mixture is smooth, turn the heat up to medium-high and bring to a boil, stirring frequently.
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3
Cook, stirring frequently, until the temperature reads 235u00b0 F on the thermometer. Stirring prevents the mixture from scorching on the base of the pan during cooking. When the mixture reaches temperature, stir in the vanilla.
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4
Pour the mixture into the buttered mixer bowl. Attach the candy thermometer to the bowl, making sure the base is as submerged as possible in the mixture. Dot the surface of the fudge with the remaining 2 tablespoons butter. Do not mix to combine.
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5
Allow the mixture to cool, undisturbed (absolutely no stirring!) until it reads 120u00b0 F on the thermometer. Remove the thermometer and attach the bowl to an electric mixer fitted with the paddle attachment.
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6
Beat the mixture on medium speed until it has lightened slightly in color and has lost some of its sheen (it will look more matte). It should hold its shape when dropped from the paddle into the bowl. This will take about 3 to 4 minutes. (You can also do this by hand in a large bowl with a wooden spoon; it will take 4 to 6 minutes.)
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7
Pour the mixture into the prepared pan and smooth into one layer. Allow to set at room temperature until cool, 45 minutes to 1 hour. Remove the cooled fudge from the pan and cut into 1-inch squares.
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8
Store the fudge in airtight containers (or wrap the pieces individually) to prevent it from drying out. It can be stored at room temperature for up to 2 weeks, or frozen for up to 3 months. Thaw before serving.
