Blueberry Cake

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.

For the blueberry reduction:
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 ⅓ cup, 8 to 10 minutes. Set aside.

For the cake:
Whisk milk and vinegar into 1/4 cup blueberry reduction, until combined. Set aside.

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.

In a medium bowl, sift together flour, baking powder, baking soda, and salt. Whisk to evenly combine.

Add to the creamed mixture alternately with the milk mixture (beginning and ending with the flour), beating well after each addition.

For layer cake, bake for 30-35 minutes, or until toothpick comes out clean. For cupcakes, fill cups ⅔ 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.

For the frosting:
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.

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.