Instructions

1. Place the butter in a heatproof bowl and set it aside. Whisk together the sugar, lemon juice, egg yolks, and egg in a medium saucepan. Cook over medium heat, stirring continuously to prevent the eggs from curdling, until the mixture is thick enough to coat the back of a spoon or registers 160°F (70°C) on a candy thermometer, 6 to 8 minutes.
2. Remove the curd from the heat and strain it through a fine-mesh sieve over the bowl containing the butter. Stir to combine. (If the butter does not melt into the lemon curd, place all of the contents in the top of a double boiler. Stir until the butter is completely incorporated.)
3. Cover with plastic wrap, pressing it directly against the surface of the curd to prevent a skin from forming, and refrigerate until set, at least 4 hours or overnight.
4. Arrange a rack in the middle of the oven and heat to 350°F (175°C). Grease and flour 3 (6-inch or 15-centimeter) round cake pans and set aside.
5. Sift together the flour, baking powder, baking soda, and salt and set aside. Place the sugar and lemon zest in a small bowl. Rub them together between your fingertips until fragrant.
6. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed for 2 minutes. Add the sugar mixture and mix on medium-high until light and fluffy, 3 to 5 minutes. Stop the mixer and scrape down the bowl.
7. Turn the mixer to medium-low and add the eggs, egg whites, lemon juice, and vanilla. Stop the mixer and scrape down the bowl.
8. Turn the mixer to low and add the flour mixture in 3 batches, alternating with the buttermilk, beginning and ending with the flour mixture. Mix on medium for no more than 30 seconds after the last streaks of the dry ingredients are combined.
9. Evenly divide the batter among the prepared pans. Bake for 22 to 24 minutes, or until a toothpick inserted into the center of the cakes comes out clean. Let them cool on a wire rack for 10 to 15 minutes before removing the cakes from their pans.
10. Stir together the sugar, water, lemon juice, and zest in a saucepan.
11. Bring the mixture to a boil over medium-high heat. Reduce the heat to low and simmer for 10 minutes, or until the mixture thickens to a syrup. Remove from the heat to cool.
12. Place the egg whites and sugar in the bowl of a stand mixer. Whisk them together by hand to combine.
13. Fill a medium saucepan with a few inches of water and place it over medium-high heat. Place the mixer bowl on top of the saucepan to create a double boiler. The bottom of the bowl should not touch the water. Whisking intermittently, heat the egg mixture until it registers 145°F to 155°F (63°C to 68°C) on a candy thermometer or is hot to the touch.
14. Once hot, carefully fit the mixer bowl onto the stand mixer. With the whisk attachment, beat the egg white mixture on high speed for 8 to 10 minutes, until it holds medium-stiff peaks. When done, the outside of the mixer bowl should return to room temperature and no residual heat should be escaping the meringue out of the top of the bowl. Stop the mixer and swap out the whisk attachment for the paddle.
15. With the mixer on low speed, add the butter, a few tablespoons at a time, then the vanilla. Once incorporated, turn up the mixer speed to medium-high and beat until the buttercream is silky-smooth, 3 to 5 minutes.
16. Tint the buttercream with the food coloring of your choice, if using. Fill a pastry bag fitted with a large round tip with about 1 cup (240 milliliters) buttercream.
17. Once the cakes have completely cooled, level them and choose which layer will be at the bottom. Using a pastry brush, generously brush the tops of each cake with the lemon simple syrup. Place the bottom layer on a cake plate or serving dish.
18. Pipe a buttercream dam around the top edge of the cake. Fill in with half the lemon curd. Place the next cake on top and repeat the buttercream and lemon curd. Add any buttercream left in the piping bag to the remaining buttercream and mix together. Crumb coat and smoothly frost the cake with the buttercream.
19. Decorate with sugar pearls, if using: randomly scatter sugar pearls around the base of the cake. Working with a small handful at a time, gently press the pearls into the fresh buttercream if they do not stick on their own. Alternatively, place the pearls on the top edge of the cake and let them trickle down the sides to create a graduated effect.