Blueberry Cake

Directions

1. 1
   Adjust oven rack to middle position and heat oven to 350°F. Grease and flour two 9-inch cake pans or line a regular 12 cup muffin tin with paper cups.
2. 2
   For the blueberry reduction:
3. 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.
4. 4
   For the cake:
5. 5
   Whisk milk and vinegar into 1/4 cup blueberry reduction, until combined. Set aside.
6. 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.
7. 7
   In a medium bowl, sift together flour, baking powder, baking soda, and salt. Whisk to evenly combine.
8. 8
   Add to the creamed mixture alternately with the milk mixture (beginning and ending with the flour), beating well after each addition.
9. 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.
10. 10
    For the frosting:
11. 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.
12. 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.