• Preheat the oven to 325 degrees F. Butter 3 8" round cake pans.
• Line with parchment paper and butter the parchment paper. Dust the cake pans with flour and knock out any excess flour.
• Whisk together the flour, baking soda, baking powder, and salt. Set aside.
• In the bowl of an electric mixer, fitted with the paddle attachment, cream the butter and shortening together, about 4 minutes. Add the sugar, vanilla, and lemon zest. Beat on medium speed until fluffy.
• Scrape down the sides of the bowl with a rubber spatula.
• Add the egg, and beat until just combined. Reduce the speed to low.
• Add the dry ingredients, alternating with the ice water, in three additions. Scrape down the sides and mix on low to make sure everything was incorporated.
• In a clean bowl, whisk egg whites and cream of tartar until soft peaks form. Be careful no to over beat.
• Gently fold the egg whites into the batter.
• Divide the batter among the prepared pans and smooth the tops.
• Bake for 40 to 45 minutes, rotating the pans halfway through the baking time, until a toothpick inserted comes out clean.
• Transfer the cakes to a wire rack and cool for 20 minutes before turning out and cooling completely.

Make the Lemon Curd Filling:

• In a small bowl, pour the lemon juice over the lemon zest and let stand for 10 minutes. In a nonreactive bowl, whisk together the eggs, egg yolks, and sugar until combined.
• Add the lemon zest and and lemon juice to the egg 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.
• Remove the bowl from the heat and whisk in 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 he curd does not form a skin.
• Do not refrigerate the curd unless you are 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 and has thickened, about 20 minutes. (At about the 10 minute mark things start to change and thicken a bit. At the 15 minute mark, start really paying attention to prevent burning your frosting).
• Transfer the mixture to the bowl of an electric mixer fitted with the paddle attachment.
• Cover with a clean dish towel and beat on high until cool. Reduce the speed to low and add the butter; mix until thoroughly incorporated. Increase the speed to medium-high and beat until light and fluffy.
• Add the vanilla and 1/2 cup lemon curd and mix until combined. Go on, take a taste test. If the frosting is too soft, refrigerate it until it is the proper consistency. If too firm, place the bowl over a pot of simmering water and beat with a wooden spoon until.

Assemble the cake:

• Refrigerate the frosting for only a few minutes until it can hold its shape.
• Place one cake layer on a serving platter and trim the top, if needed, to create a flat surface.
• Spread about 1 cup of the lemon curd evenly on top. Add the next layer and trim if needed.
• Add the remaining lemon curd and smooth. Top with the last cake and trim if needed.
• Crumb coat the cake and refrigerate for about 15 minutes to set.
• Frost the sides and top of the cake with the frosting.
• Garnish with lemon drop candies and place in refrigerator for 15 minutes to set.