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.