Prepare the Blueberry Powder: Process the freeze-dried blueberries in a blender or food processor until finely powdered, then sift the powder to remove the seeds.
Make the Buttercream: The night before assembling the cake, beat the softened butter, salt, vanilla, and blueberry powder together in a stand mixer or with a hand mixer. Gradually add the powdered sugar, sifting it first if lumpy, and beat for at least 3 minutes, scraping down the bowl as needed. Add milk or heavy cream as necessary until the buttercream becomes smooth. Rest the Buttercream: Transfer the buttercream to an airtight container and leave it at cool room temperature overnight. The blueberry color will deepen as the blueberry particles dissolve. Stir the buttercream thoroughly before using to blend and intensify the color. Prepare the Blueberry Filling: Combine the blueberries, sugar, lemon zest, and lemon juice in a small pot. Cook over medium heat for about 15 minutes, stirring as needed, until the berries soften and burst. Transfer the filling to a container and refrigerate until cooled and thickened. Reduce the Fresh Blueberries: Puree the 1½ cups fresh blueberries in a blender or food processor. Pass the puree through a sieve to remove the seeds.Transfer it to a pot and cook over medium heat until reduced, thickened, and much of the water has evaporated. The recipe requires ½ cup of blueberry reduction for the cake. Allow it to cool completely before adding it to the batter. Prepare the Cake Pans: Preheat the oven to 325°F. Butter and flour two 8-inch round cake pans.
Create the Cake Batter: Place the softened butter, sugar, cake flour, baking powder, baking soda, and salt in the bowl of a stand mixer fitted with the paddle attachment. Beat until everything is fully incorporated and the mixture has a sandy texture. Add the Eggs: Add the room-temperature eggs one at a time, allowing each egg to become fully incorporated before adding the next. Once all four eggs have been added, continue mixing for a few minutes, scraping the bottom and sides of the bowl as necessary. Add the Blueberry Flavor: With the mixer running, slowly pour in the cooled blueberry reduction, blueberry powder, lemon juice, and kefir. Mix until the batter is completely combined.
Fill the Cake Pans: Divide the finished batter evenly between the two prepared 8-inch cake pans and smooth as needed.
Bake the Cake: Bake at 325°F for about 40 minutes, or until the tops feel firm when gently pressed or a cake tester inserted into the cakes comes out clean.
Cool the Cake Layers: Allow the baked cakes to cool completely before beginning the assembly.
Prepare the Buttercream for Assembly: Stir the rested buttercream thoroughly with a rubber spatula until its color is evenly blended and intensified. Transfer the buttercream to a large piping bag.
Trim the Cake Layers: Use a bread knife to carefully slice the domed tops from the cooled cakes so the layers are level.
Create the Filling Border: Place one cake layer on your work surface. Pipe a border of buttercream around its top edge, creating a wall to contain the blueberry filling.
Add the Filling: Spread the blueberry filling inside the buttercream border. On the second cake layer, spread a thin layer of buttercream across the surface.
Stack the Cake: Carefully place the second cake layer face down over the filled bottom layer.
Frost the Cake: Pipe the remaining buttercream around the sides and over the top of the assembled cake. Use an offset spatula to smooth the buttercream into an even finish.
