Generously grease and flour the bottom and sides of two - 8" or 9" round cake pans; set aside. Mix the flour, baking powder, and baking soda together; set aside. In a separate bowl, using an electric mixer at medium speed, mix the butter and sugar together for 3 to 4 minutes , or until light and fluffy. Add the eggs, one at a time, beating well after each addition, then beat for an additional 3 to 5 minutes or until the batter is smooth, thick, and creamy. Slowly add the flour mixture, alternately with the buttermilk; while beating at low speed. Add the vanilla and lemon zest and beat again. Pour the batter into the the prepared cake pans. Bake in a preheated oven at 350°, for 50 to 55 minutes or until a pick inserted into the center comes out clean. Cool on a wire rack, in the pans, for about 10 minutes, then remove the cakes from the pans and continue to cool on wire racks.
While the cakes are baking... Whisk the egg yolks and whole eggs together. Pour into a small saucepan. Add the sugar and lemon juice. Cook over low heat, while stirring almost constantly with a wooden spoon. Continue to cook for 8 to 10 minutes or until the mixture is slightly thickened and cto the spoon. Remove from heat and let cool for about 20 minutes. Place a strainer over a small mixing bowl and strain the filling mixture. Cut the butter into small pieces, then slowly stir it into the filling mixture. Stir until smooth and well blended. Allow to cool completely.
Combine the egg whites, vanilla, and cream of tartar in a deep mixing bowl. Gradually increasing the mixer speed to high, beat until soft peaks form. Add the sugar, a small amount at a time, beating on high speed until the mixture forms glossy, stiff peaks.
Classic Cake Stand with Cover