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1
Adjust oven rack to middle position and heat oven to 350u00b0F. Grease and flour two 9-inch cake pans or line a regular 12 cup muffin tin with paper cups.
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2
For the blueberry reduction:
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3
Transfer blueberries to bowl, cover, and microwave until blueberries are soft and have released their juice, about 5 minutes. Place in fine-mesh strainer set over small saucepan. Firmly press fruit dry; discard blueberry solids. Bring juice to boil over medium-high heat and cook, stirring occasionally, until syrupy and reduced to 1/3 cup, 8 to 10 minutes. Set aside.
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4
For the cake:
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5
Whisk milk and vinegar into 1/4 cup blueberry reduction, until combined. Set aside.
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6
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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7
In a medium bowl, sift together flour, baking powder, baking soda, and salt. Whisk to evenly combine.
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8
Add to the creamed mixture alternately with the milk mixture (beginning and ending with the flour), beating well after each addition.
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9
For layer cake, bake for 30-35 minutes, or until toothpick comes out clean. For cupcakes, fill cups 2/3 full and bake 20-22 minutes or until a toothpick inserted in the center comes out clean. Let cool in pan for 10 minutes then remove to a wire rack to cool completely.
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10
For the frosting:
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11
Using a stand mixer fitted with a paddle, mix butter and cream cheese on medium speed until combined, about 30 seconds. Add confectioners' sugar and mix on low speed until incorporated. Increase speed to medium-high and beat until pale and fluffy, about 2 minutes.
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12
Add remaining blueberry reduction and salt and mix until combined, about 30 seconds. Add additional 1 cup confectioners' sugar to thicken frosting as desired. Refrigerate until ready to use, up to 2 days.
