For the lemon curd:
5 tablespoons (70 grams) unsalted butter, diced, at room temperature
3/4 cup (150 grams) granulated sugar
5 tablespoons (75 milliliters) fresh lemon juice
2 large egg yolks
1 large egg
For the light lemon cake:
Butter or nonstick cooking spray, for the pans
2 1/4 cups (295 grams) cake flour, plus more for the pans
1 1/2 teaspoons baking powder
3/4 teaspoon baking soda
1/4 teaspoon fine salt
1 1/2 cups (300 grams) granulated sugar
1 tablespoon finely grated lemon zest
3/4 cup (1 1/2 sticks/170 grams) unsalted butter, at room temperature
3 large eggs
2 large egg whites
2 tablespoons fresh lemon juice
1 teaspoon pure vanilla extract
1 cup (240 milliliters) buttermilk
For the lemon simple syrup:
1/2 cup (100 grams) granulated sugar
1/4 cup water
2 tablespoons fresh lemon juice
2 teaspoons finely grated lemon zest
For the buttercream:
1/2 cup (120 milliliters) large egg whites
1 cup (200 grams) granulated sugar
1 1/2 cups (3 sticks/340 grams) unsalted butter, at room temperature
1 1/2 teaspoons pure vanilla extract
For assembly:
Gel food coloring (optional)
Sugar pearls (optional)
Make the lemon curd: 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.
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.)
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.
Make the light lemon cake: 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.
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.
In the bowl of a stand mixer fitted with the paddle