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1
Adjust oven rack to middle position. Preheat oven to 350u00b0F and line a regular 12 cup muffin tin with paper cups.
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2
Transfer strawberries to a bowl, cover and microwave until strawberries are soft and have released their juice, about 5 minutes.
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3
Place berries in a fine-mesh strainer set over a small saucepan. Firmly press the liquid out of the fruit until fruit is dry (juice should measure at least 3/4 cup). Reserve strawberry solids in the refrigerator.
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4
Bring juice to a boil over medium-high heat and cook, stirring occasionally, until syrupy and reduced to 1/4 cup. This will take about 6 to 8 minutes. Whisk milk and vinegar into juice until combined; set aside.
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5
In the bowl of your stand mixer cream together butter and sugar until light and fluffy, about 3-4 minutes. Add egg whites, one at a time, beating well after each addition. Mix in vanilla.
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6
In a medium bowl, sift together flour, baking powder, baking soda, and salt. Whisk to evenly combine. Add this into the creamed mixture alternately with 1/4 cup of the milk mixture (beginning and ending with the flour), beating well after each addition.
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7
Fill cups 2/3 full and bake 20-22 minutes or until a toothpick inserted in the center comes out clean. Remove pan from the oven and set it on a rack. Let cupcakes cool in pan for 10 minutes then remove them to a wire rack to cool completely.
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8
For the frosting: In the clean bowl of your stand mixer fitted with paddle attachment, mix butter and 3 cups confectioners' sugar on low speed until combined, about 30 seconds.
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9
Increase speed to medium-high and beat until pale and fluffy, about 2 minutes. Add cream cheese, one piece at a time, and beat until incorporated, about 1 minute.
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10
Add the reserved strawberry solids and salt and mix until combined, about 30 seconds. Add additional 1 cup of confectioners' sugar as needed to thicken frosting. Refrigerate until ready to use, up to 2 days.
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11
Pipe or spread frosting onto cooled cupcakes.
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12
Method and frosting recipe modified from Cooks Country.
