1 and 1/4 cups fresh blueberries , plus more for topping
Cream Cheese Frosting:
8 oz cream cheese , softened
1/2 cup butter , softened (I prefer 1/4 cup unsalted and 1/4 cup salted)
1/2 tsp vanilla extract
2 cups powdered sugar
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.
In the bowl of an electric stand mixer fitted with the paddle attachment (I recommend using a beater blade, otherwise stop mixer occasionally throughout entire mixing process and scrape down sides and bottom of bowl), whip butter and granulated sugar until pale and fluffy. Mix in egg, then mix in egg whites one at a time. Blend in vanilla. 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.
Divide batter among 12 paper lined muffin cups, filling each about 3/4 full (about 1/3 cup batter in each). 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.
For the Cream Cheese Frosting:
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).
*Just thought I'd throw it out there, if you only have salted butter that will be fine for this recipe, simply reduce the salt to 1/4 tsp. Also, when softening your butter, you can either leave it out to nearly reach room temp, or you can heat it in the microwave, rotating it about every 4 seconds until soft (so it softens evenly). Just don't let it melt.