Step 1

Preheat oven to 350 degrees. Sift cake flour into a mixing bowl, add baking powder, baking soda and salt and whisk 20 seconds, set aside.

Step 2

In the bowl of an electric stand mixer fitted with the paddle attachment, whip butter and granulated sugar until pale and fluffy. Scrape down bowl. Mix in egg, then mix in egg whites one at a time. Blend in vanilla.

Step 3

In the liquid measuring cup used to measure buttermilk, whisk together sour cream with buttermilk until smooth. Working in three separate batches, beginning and ending with flour mixture, add 1/3 of the flour mixture alternating with 1/2 of the buttermilk mixture and mixing just until combined after each addition. Gently fold in blueberries.

Step 4

Divide batter among 12 paper lined muffin cups, filling each about 3/4 full (about 1/3 cup batter in each).

Step 5

Bake in preheated oven 20 - 24 minutes until toothpick inserted into center comes out clean. Allow to cool in muffin tin several minutes, then transfer to a wire rack to cool completely. Once cool, pipe or spread cream cheese frosting over tops and garnish with blueberries.

Step 6

For the Cream Cheese Frosting:

Step 7

In the bowl of an electric stand mixer fitted with the paddle attachment (I recommend using a beater blade), whip cream cheese and butter until smooth and fluffy. Mix in vanilla, then stir in powdered sugar and continue to whip until smooth and fluffy (if it seems slightly runny, chill then stir before piping or spreading).