Preheat oven to 350F. Prep 3 six-inch pans by spraying with oil, then lining the bottoms with circles cut out of parchment paper. In the bowl of an electric mixer fitted with the paddle attachment, cream together the butter and 2 cups of sugar until light and fluffy, about 5 minutes. On medium speed, add the eggs one at a time until incorporated, then add the zest. In a separate bowl, sift together the flour, baking powder, baking soda, and salt. In a different bowl, whisk together the buttermilk, vanilla extract, and 1/4 cup lemon juice. With your mixer on med-low, add the flour mixture and buttermilk mixture alternately to the batter, beginning and ending with the flour. Fill each pan halfway, and bake about 35 minutes or until a toothpick inserted comes out clean. Let cool in the pans for about 30 minutes before removing. In a small saucepan, combine the leftover 1/2 cup sugar and 1/2 cup lemon juice. Place over low heat, and stir until sugar is dissolved. Slice the rounded tops off of each cake layer to make them level, and brush the lemon syrup onto each layer. Let cool completely.

In a heavy-bottomed saucepan, whisk together the juice, zest, sugar and egg yolks. Place over med-low heat, and add the butter. Whisk frequently until the mixture is thick enough to leave a trail when the whisk is run through it (10-15 minutes). Remove to a clean container, and chill for at least an hour.

Fill a saucepan with 1-2 inches of water and bring to a simmer. Combine the egg whites and sugar in a heatproof bowl, and place the bowl on top of the simmering saucepan. Whisk until the sugar is dissolved completely. Transfer the egg whites to the bowl of an electric mixer (make sure the bowl is squeaky clean). Whip the egg whites until they double in size. Add vanilla and beat to combine. Then, on low speed, add the butter one stick at a time. Continue beating until the frosting is smooth.

On two cake layers, spread a thin layer of raspberry jam, stopping about half an inch from the edge of the cake. On top of that, spread a thin layer of lemon curd. Layer the two cakes on top of each other, then turn your last layer upside down and place it on the top. Use an offset spatula to cover the cake with buttercream, and enjoy!