For the Cake:

1. Preheat oven to 350°F. Grease and flour three 6-inch round cake pans. Wrap pans with damp cake strips for even baking, if desired.
2. In a medium bowl, whisk together milk, sour cream, egg whites, lemon juice, lemon zest, and vanilla. Set aside.
3. In another bowl, sift together flour, sugar, baking powder, and salt. Remove 1 tablespoon of this mixture and toss it with the blueberries in a small bowl. Set aside.
4. In a stand mixer fitted with the paddle attachment (or a large mixing bowl), beat softened butter on medium speed until creamy, about 1 minute. Add the dry ingredients and mix on low speed until the mixture resembles sand. Scrape down the bowl as needed.
5. With the mixer on low, slowly pour in the wet ingredients. Mix until just combined. Scrape down the bowl, then beat on medium-low speed for 30 seconds until fluffy. Gently fold in the flour-coated blueberries with a spatula.
6. Divide the batter evenly between the three pans. Bake for about 30 minutes, or until the centers spring back when touched. Let cool in pans for 15 minutes, then turn out onto a wire rack to cool completely.

For the Blueberry Reduction:

1. Freeze ½ cup of blueberries.
2. In a small saucepan, combine the remaining 1 cup of blueberries with sugar, lemon juice, and water over medium-low heat. Cook until berries burst, then mash and simmer until reduced by half.
3. Strain the mixture through a fine-mesh sieve, pressing out the liquid. Discard solids and chill the reduction until ready to use.

For the Lemon Buttercream:

1. In a stand mixer with paddle attachment, beat butter and salt on medium speed until fluffy, about 3 minutes.
2. Reduce to low speed and gradually add confectioners’ sugar, scraping the bowl as needed. Once incorporated, beat on medium speed for 1 minute until smooth.
3. Add lemon juice one tablespoon at a time until desired consistency and flavor is reached.
4. Transfer 1 cup of buttercream to a small bowl. Mix in blueberry reduction 1 tablespoon at a time to create blueberry buttercream.
5. Transfer blueberry frosting to a piping bag (snip the tip). Transfer ½ cup of lemon frosting to a piping bag fitted with tip 846. Place the remaining lemon buttercream in a separate piping bag with the tip snipped off.

To Assemble:

1. Place one cake layer on a stand. Pipe a ring of lemon buttercream around the edge. Fill the center with blueberry buttercream.
2. Add another cake layer and repeat.
3. Cover the top and sides with lemon buttercream. Use an offset spatula to smooth the frosting. Scrape the sides for a semi-naked look if desired.
4. Pipe a ring of lemon buttercream dollops 1 inch from the cake’s edge. Fill the center with the frozen blueberries.
5. Chill the cake for 20 minutes before slicing.