Preheat oven to 350°F. Butter and flour 2 (9-inch) round cake pans and set aside.
Beat the butter in a stand mixer or with a hand mixer at medium speed until fluffy. Add the sugar and beat well. Add the egg yolks one at a time, beating well to incorporate after each addition.
In a separate bowl, whisk together the flour and baking powder. Add one third of the mixture to the butter and beat on low until well combined. Add 1/2 of the milk, beat 'til well combined. Repeat with another 1/3 of the flour mixture, then the other half of the milk and the vanilla extract, then the remaining third of the flour mixture, beating well after each addition and scraping the bowl down as needed.
Beat the egg whites and salt to stiff peaks. Stir 1/3 of the egg whites into the cake batter, to lighten it. Then fold in the remaining egg whites.
Divide the batter between the two cake pans. Smooth the tops. Bake for 30-38 minutes, or until a straw inserted in the center of the cake comes out clean and the cakes springs back when pressed lightly in the center with your fingertip.
Cool in the pans on a wire rack for 10 minutes before turning out onto the rack to finish cooling completely.
When the cakes are absolutely cooled, you can trim them to level them. Place three or four strips of parchment or waxed paper over a cake plate to protect the plate while frosting and garnishing the cake. Place a trimmed, level cake in the center of the cake.
Add the lemon curd to the center of the cake and spread to within 3/4-inch of the edges. Place the second trimmed, leveled cake over the first. Cover with plastic wrap and chill for at least an hour to help it set up.