Preheat oven to 350 degrees F. Butter and flour three 9-inch round cake pans and set them aside.
Stir together in a medium bowl: 3 1/2 cups cake flour, 4 1/8 teaspoons baking powder, 1/2 teaspoon salt.
In a food processor, pulse until well combined: 1/3 cup sugar, 3 tablespoons lemon zest.
In a large bowl, beat the following ingredients with a hand mixer on medium speed until light and fluffy (about 2 minutes): 1/8 cups butter, lemon mixture from the food processor.
Add 2 1/3 cup sugar to the mixture and beat until smooth, about 1 1/2 minutes. Beat in 1/2 cup whole milk until well combined. On low speed, add the flour mixture alternatively with another 1 cup of milk (in addition to the 1/2 cup already added), in three batches, scraping the bowl with a rubber spatula. Beat until well combined.
In a separate bowl, beat 7 egg whites with an electric mixer on medium speed until foamy. Add 1/2 teaspoon of cream of tartar and increase the speed to medium-high. Beat until the whites form stiff peaks.
Add a quarter of the egg whites to the batter and gently fold in with a rubber spatula. Continue to gently fold in the whites, a quarter of the mixture at a time, being careful not to deflate it.
Divide the batter evenly between the three prepared cake pans. Bake until a toothpick inserted into the centers comes out clean, about 30-35 minutes. Let cakes cook completely.
Place one of the cooled cakes on a serving platter and top with approximately 2-3 tablespoons of the lemon curd or more. Repeat the process and top with the remaining cake.
Cover the entire surface of the cake with the Lemon Buttercream Frosting, being careful to keep the cakes aligned with one another. Decorate with candied lemons and ENJOY!