Preheat your oven to 325 degrees F. Spray three 8-inch round cake pans or four 6-inch round cake pans with nonstick spray, line the bottoms with parchment paper and spray again. Set aside.

In a medium-size mixing bowl, combine the flour, salt and baking powder. Whisk to combine. Set aside.

In a small mixing bowl, combine the buttermilk and lemon juice. Set aside.

In a 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.

Scrape down the sides of the bowl and begin to add the egg whites, one at a time, scraping down the sides and the 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.

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. Over mixing your cake batter will cause it to sink. ***As you add the last of the buttermilk mixture, this is a good time to start coloring your batter if you'd like it a brighter yellow.

Coat your blueberries in a tablespoon of flour and then gently fold them into the cake batter.

Divide the batter evenly between the three pans, about 18 ounces in each 8-inch pan or about 13 ounces in each 6-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 a few moist crumbs on it. Allow the cake layers to cool in the pans on wire racks for about 10 to 15 minutes. Invert cake layers onto a wire rack to cool completely. TIP: spray your wire rack with nonstick spray to prevent the cake layers from sticking.

Allow to cool completely before wrapping in plastic wrap and storing in the freezer until ready to decorate.

This element is best done a day or two ahead of stacking and decorating your cake.

Whisk the sugar, lemon zest, and lemon juice in a medium saucepan. Whisk in the whole eggs and yolks in a small bowl and then whisk them into the lemon mixture.

Cook the mixture over medium-low heat, whisking constantly, until it's thick like pudding, about 6 to 8 minutes. Remove from the heat and whisk in the butter, a few pieces at a time until 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. Refrigerate until completely set, at least 4 hours and up to 5 days. 

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter until light and fluffy, about 2 minutes.

Turn the mixer to low and gradually add the powdered sugar, followed by the cream, salt and lemon extract. Mix until combined. Add yellow food coloring, if using. Mix until incorporated completely. Turn the mixer to medium-high and beat the frosting for about 5 minutes.

Before applying the buttercream to your cake, use wooden spoon to beat the frosting by hand for about 2 minutes to push out any air pockets. 

Spread a small dollop of frosting onto a cake board. Place the first cake layer, top side up, on the board.

Spread about a half cup of frosting over the cake layer and pipe a rim of frosting around the edge of the cake to create a barrier to the filling.

Spread about 1/4 cup of the curd in the center of the cake, carefully spreading it to the edge of the piping.

Place a second cake layer on top of the piped frosting and filling and repeat step 2 and 3. Place the final cake layer, top side up, on the second layer of filling.

Lightly coat the cake with a thin layer of frosting to lock in the crumbs. Freeze the cake for about 10 to 15 minutes to set the frosting.

Finish frosting and decorating the cake with the remaining frosting. Serve a spoonful of the remaining lemon curd with each slice of cake.