Preheat the oven to 350°F (175°C). Grease and line two 8-inch round cake pans with parchment paper. In a bowl, whisk together the flour, baking powder, and salt.

In a large bowl, beat the butter and sugar together until light and fluffy, about 3–4 minutes. Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

Alternate adding the dry ingredients and buttermilk into the butter mixture, beginning and ending with the dry ingredients. Mix until smooth and well combined.

Divide the batter evenly between the pans. Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean. Let cakes cool in the pans for 10 minutes, then transfer to a rack to cool completely.

In a small saucepan, whisk together the egg yolks, sugar, lemon juice, zest, and salt. Cook over medium-low heat, stirring constantly until thickened (about 8–10 minutes). Remove from heat and whisk in the butter, one piece at a time, until smooth. Strain the curd if desired, then cool completely.

In a heatproof bowl, whisk together egg whites, ½ cup sugar, and water. Set over a pot of simmering water (double boiler) and whisk constantly until sugar dissolves and the mixture is warm to touch.

Transfer the mixture to a mixing bowl. Beat on high speed until stiff peaks form and the bowl is cool. Gradually add the remaining ½ cup sugar and beat again.

Add the softened butter, a few tablespoons at a time, and continue beating until the frosting is smooth and creamy. Beat in the vanilla extract.

Place one cake layer on a serving plate. Spread a generous layer of lemon curd over it. Top with the second cake layer, then cover the entire cake with buttercream frosting. Decorate as desired and chill slightly before serving.