Preheat your oven to 325 degrees. Spray three 8-inch round cake pans with nonstick spray, line the bottoms with parchment paper, and spray again. Set aside.
In a medium-sized mixing bowl, whisk the flour, salt, and baking powder. Whisk to combine. Set aside.
In the bowl of an electric mixer fitted with a paddle attachment, cream the butter, sugar and zest together on medium speed, until light and fluffy, about 2 to 3 minutes.
1 cup unsalted butter, 1 cup granulated sugar
Scrape down the sides of the bowl and add the egg whites and whole eggs, one at a time, scraping down the sides and bottom of the bowl and mixing well between each addition. Mix on medium-high speed until no grains of sugar remain, about 2 to 3 minutes.
3 large egg whites, 2 large whole eggs
Add the lemon emulsion and the sour cream. Mix on low. If it starts to look a little curdled, don’t stress. It will smooth out once you add the flour mixture.
3 teaspoons lemon emulsion, ⅓ cup sour cream
With the mixer on low speed, alternately add the flour mixture and the buttermilk mixture, starting and ending with the flour mixture. Be sure not to over mix.
1 ¼ cups buttermilk
Divide the batter evenly between the three pans, about 18 ounces in each 8-inch pan. Bake the cake layers for 25 to 30 minutes, or until a toothpick inserted in the center of the cakes comes out with only a few moist crumbs on it. Allow the cake layers to cool in the pans and then transfer them onto a wire rack to finish cooling.
Allow to cool completely before wrapping in plastic wrap and storing in the freezer or fridge (they hold in the fridge for 3 days) or until ready to decorate.
For the lemon curd
I suggest doing this section of the cake a day before to ensure that it sets properly.
In a medium saucepan, whisk the sugar, lemon zest, and lemon juice. in a small bowl, whisk in the whole eggs and yolks until combined, and then whisk them into the lemon mixture.
1 cup granulated sugar, 1 Tablespoon lemon zest, ½ cup fresh lemon juice, 3 large eggs, 4 egg yolks
Whisking constantly, start cooking the mixture over medium-low heat until it's thick like pudding, about 6 to 8 minutes. It will start to bubble, keep whisking and cook for about 30 more seconds Remove from the heat and whisk in the butter until fully incorporated.
Strain the mixture through a fine-mesh sieve into a small bowl, pushing it through with a rubber spatula. Press plastic wrap directly onto the surface to prevent a skin from forming. Let it come to room temperature and then refrigerate.
4 Tablespoons unsalted butter
For the blueberry buttercream
In the bowl of a stand mixer fitted with a paddle attachment, soften the butter by creaming it for about 2 minutes.
2 cups unsalted butter
With the mixer on low speed, add the sifted powdered sugar.
5 cups powdered sugar
Add the freeze dried blueberries and blueberry emulsion.
2 teaspoons blueberry emulsion, ¼ cup freeze dried bluberries
With mixer on medium speed, add the whipping cream and salt.
⅓ cup heavy whipping cream
Turn the mixer to medium-high speed and beat for an additional 5 minutes. The frosting will become lighter in color and texture. With a rubber spatula, beat by hand to beat out any air pockets.
Assembly
With a piping bag, pipe a small dollop of frosting onto a cake board. Place the first lemon cake layer, top side up, onto the board.
Pipe a rim of frosting around the edge of the cake to create a barrier for the filling.
Spread about half of the curd in the center of the cake. With the back of a spoon, carefully spread it to the edge of the piping.
Place a second cake layer on top of the piped frosting and filling and repeat steps 2 and 3. Place the final cake layer, top side up, on the second layer of filling.
Frost a thin coat of frosting around the cake to lock in the crumbs. This is the crumb coat. Freeze the cake for about 10 to 15 minutes to set the frosting.
Finish frosting with the blueberry buttercream, and decorating the cake with the remaining frosting. Serve with the remaining lemon curd.