1-Make Lemon Curd: In a medium saucepan, combine lemon juice, lemon zest, sugar, salt, and egg yolks. Cook over medium-low heat, whisking constantly for 20-25 minutes until thick enough to coat the back of a spoon. Remove from heat and whisk in cold cubed butter until smooth. Strain curd to remove lumps, cool completely, and refrigerate until ready.

2-Prepare Cake Batter: Preheat oven to 350°F (175°C). Grease and line three 8-inch round pans or one 9×9 pan. In a bowl, whisk together flour, baking powder, baking soda, and salt. In a separate bowl, cream softened butter and sugar for 2-3 minutes until pale and fluffy. Gradually add egg whites, mixing after each addition until smooth. Add vanilla extract and lemon zest. Alternate adding dry ingredients and buttermilk (plus oil if using), beginning and ending with dry ingredients. Mix on low until just combined without overmixing.

3-Bake: Divide batter among pans and bake 33-40 minutes until a toothpick inserted in center comes out clean. Cool in pans 10-15 minutes, then transfer to wire racks to cool completely. Optionally level or tort layers for thinner cake layers.

4-Make Frosting: Cream Cheese Frosting: Beat softened butter and cream cheese until smooth. Gradually add powdered sugar and mix well. Add lemon zest and juice, then beat until light and fluffy. Adjust consistency with cream if desired. Swiss Meringue Buttercream: Heat egg whites and sugar to 160°F (70°C) over simmering water, whisking constantly. Beat until stiff glossy peaks form. Gradually add softened butter, lemon zest, juice, vanilla, optional lemon extract, and salt, mixing until smooth and spreadable.

5-Assemble Cake: Place a cake layer on serving plate. Pipe a thick border of frosting around edge, spoon or pipe about 1/3 cup lemon curd in center. Add next layer and repeat. Stack final layer and frost cake evenly. Optionally swirl lemon curd on top and garnish with lemon zest or thin lemon slices.