Preheat oven to 325 degrees. Butter 3 8 round cake pans, line the bottoms with parchment, and butter the parchment. Dust with flour and and knock out.
In the large bowl, sift the flours, baking powder, baking soda, and salt together. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and shortening on medium speed until creamy, 3 to 4 minutes. Add the sugar, vanilla, and lemon zest and beat on medium speed until fluffy, about 3 minutes. Scrape down the sides of the bowl, add the egg, and beat until just combined. Reduce speed to low. Add the flour mixture, alternating with the ice water, in three separate additions, beginning and ending with the flour mixture. Scrape down the bowl, then mix on low speed for a few more seconds.
In a clean bowl, whisk the egg whites and cream of tartar until soft peaks form. Do not overbeat. Gently fold the egg whites into the batter.
Divide the batter among the prepared pans and smooth the tops. Bake for 40-45 minutes, rotating the pans halfway through baking time, until a toothpick inserted in the center of each cake comes out clean. Transfer the cakes to a wire rack and let cool for 20 minutes. Invert the cakes onto the rack, remove pans, and let cool completely. Remove the parchment.
Make the Lemon Curd Filling: In a small bowl, pour the lemon juice over the lemon zest and let stand for 10 minutes to soften the zest.
In a nonreactive bowl whisk the eggs, egg yolks, and sugar until combined. Add the lemon zest and lemon juice to the mixture and whisk until just combined.
Place your bowl containing the egg mixture over a double boiler. Continuously stir the mixture with a heatproof spatula until the mixture has thickened to a pudding-like texture, about 6 minutes (this took me more like 20 minutes, on high heat at that).
Remove the bowl from the heat and whisk the butter until emulsified. Strain the mixture through a fine-mesh sieve. Take a sheet of plastic wrap and press it into the mixture and around the bowl so that the curd does not form a skin.
Set the lemon curd aside while you are making the frosting. Do not refrigerate the curd unless you're saving it for future use.
Make the Lemon Drop Frosting: In a medium heavy-bottomed saucepan, whisk the sugar and flour together. Add the milk and cream and cook over medium heat, whisking occasionally, until the mixture comes to a boil & has thickened, about 20 minutes.
Transfer the mixture to the bowl of an electric mixer fitted with a paddle attachment. Beat on high speed until cool. Reduce the speed to low and add the butter; mix until thoroughly incorporated. Increase the speed to medium-high and beat until the frosting is light and fluffy.
Add the vanilla extract and 1/2 cup of the lemon curd and continue mixing until combined. If the frosting is too soft, put it in the refrigerator to chill it slightly then mix it again until it is the proper consistency. If the frosting is too firm place the bowl over a pot of simmering water and beat with a wooden spoon until it is the proper consistency.
Assemble the Cake: Refrigerate the frosting for a few minutes (but no more) until it can hold its shape. Place one cake layer on a serving platter. Trim the top to create a flat surface and evenly spread about one cup of the remaining lemon curd filling on top. Add the next layer, trim and fill with 1 cup of lemon curd, then add the third layer and trim. Crumb coat the cake and refrigerate for about 15 minutes. Frost the sides and the top of the cake with the frosting. Garnish with the candies and refrigerate for 15 minutes to firm up the finished cake.
This cake will keep beautifully in a cake saver at room temperature (cool and humidity free) for up to 3 days. If your room is not cool, place the cake in a cake saver & refrigerate for up to 3 days.Remove the cake from the refrigerator and let it sit at room temperature for at least 2 hours before serving.