Preheat the oven to 350°F. Butter and flour three 6-inch round cake pans.

Beat together the wet ingredients, including the milk, sour cream, egg whites, lemon juice and zest, and vanilla, together in a medium bowl. Set aside.

Sift the dry ingredients, including the flour, sugar, baking powder, and salt, together in a medium bowl. Remove one tablespoon and add it to a small bowl with the blueberries. Toss the berries to coat in flour and set aside.

In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter on medium speed until creamy, about 1 minute. Add the flour mixture and beat on low speed until the mixture is sandy. Stop and scrape down the bowl as needed.

With the mixer running on low speed, slowly pour in the wet ingredients and mix until just combined. Scrape down the bowl and beat on medium-low speed until fluffy, about 30 seconds. Fold in flour-coated blueberry by hand with a spatula.

Divide the mixture evenly into the cake pans.

Bake for about 30 minutes or until the centers are springy to the touch. Let the cakes cool in the pans for 15 minutes, then remove them and place them on a wire cooling rack to cool completely.

Place ½ cup of blueberries in the freezer.

In a small saucepan over medium-low heat, combine the remaining 1 cup of blueberries, sugar, lemon juice, and water. Cook until the berries start to burst. Mash and stir for a minute, then simmer until reduced by half.

Pour the blueberry reduction into a fine-mesh strainer and press through, catching the liquid in a small bowl. Discard the remaining solids. Chill the reduction until ready to use.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and salt on medium speed until fluffy, about 3 minutes.

Turn the mixer to low speed and gradually add in the confectioners' sugar. Scrape the bowl down occasionally. Once incorporated, beat on medium speed until fluffy, about 1 minute.

Beat in the lemon juice a tablespoon at a time until your desired consistency and taste is reached.

Transfer about 1 cup of the frosting to a small bowl, and mix in blueberry reduction a tablespoon at a time until you reach your desired flavor.

Transfer the blueberry buttercream to a piping bag and snip off the tip. Transfer ½ cup of the lemon buttercream to a piping bag fitted with a decorative tip (846). Then, place the rest of the lemon buttercream in a plain piping bag and snip off the tip.

Pipe a thick ring or lemon buttercream around the edge of one cake layer. Pipe the blueberry-lemon buttercream onto the cake, filling the center of that ring. Top with another cake layer and repeat with the icing.

Pipe more lemon buttercream around the sides and top of the cake and smooth it using an offset spatula.

Pipe a circle of lemon buttercream dollops about 1 inch from the edge of the cake. Add frozen blueberries to the inside of the circle. Refrigerate the cake for 20 minutes before slicing.