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

In a medium bowl, beat together the milk, sour cream, egg whites, lemon juice, lemon zest, and vanilla extract. Set aside.

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

In a large mixing bowl or stand mixer, beat the softened butter on medium speed until creamy, about 1 minute. Add the flour mixture and beat on low speed until sandy.

With the mixer 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 the flour-coated blueberries by hand with a spatula.

Divide the mixture evenly into the prepared 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 and place on a wire cooling rack to cool completely.

For the blueberry reduction, place ½ cup of blueberries in the freezer. In a small saucepan over medium-low heat, combine the remaining blueberries, sugar, lemon juice, and water. Cook until the berries burst, mash and stir for a minute, then simmer until reduced by half. Strain the reduction and chill.

For lemon buttercream, in a stand mixer, beat the butter and salt on medium speed until fluffy, about 3 minutes. Gradually add confectioners’ sugar and beat until fluffy, about 1 minute. Add lemon juice until desired consistency is reached.

Transfer about 1 cup of frosting to a small bowl and mix in blueberry reduction until desired flavor is reached.

Pipe a thick ring of lemon buttercream around the edge of one cake layer, fill the center with blueberry-lemon buttercream, top with another cake layer, and repeat.

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

Pipe lemon buttercream dollops around the edge and add frozen blueberries inside the circle. Refrigerate for 20 minutes before slicing.