PINK LEMONADE CAKE

Steps:

• For the cake: Position racks in the upper and lower third of the oven and preheat to 350 degrees F. Brush the bottom and sides of four 9-inch round cake pans with vegetable oil. Dust with flour and tap out the excess; set aside. Whisk the flour, cornstarch, baking powder, baking soda and salt together in a medium bowl. Stir the milk and vanilla together in a large liquid measuring cup. Set both aside.
• Beat the butter, lemon zest and juice, vegetable oil and sugar in a large bowl using an electric mixer on medium-high speed until light and fluffy, 5 to 7 minutes, stopping to scrape down the sides of the bowl occasionally using a rubber spatula. Beat in the eggs 1 at a time until combined and then continue to beat on medium-high speed until mixture is thick, smooth and shiny, about 3 minutes more. Reduce the speed to medium-low and add one-third of the flour mixture until just combined. Beat in half the milk mixture until combined. Repeat these additions, ending with the final one-third of the flour mixture, and beating just until smooth. Stir a few times with the spatula to make sure the batter is well blended.
• Evenly divide the cake batter among 4 medium bowls. Tint one bowl with 1 drop of pink food coloring, tint another with 2 drops of food coloring, the third with 1 teaspoon food coloring, and the last with 1/4 teaspoon food coloring. Stir each bowl until no streaks remain; the frostings should have gradually lighter shades of pink.
• Transfer each bowl of frosting to one of the prepared pans. Bake until the tops are lightly browned, the edges of the cake are beginning to pull away from the side of the pan and the center lightly springs back to the touch, about 30 minutes. Cool in the pans 10 to 15 minutes, then turn out onto wire racks to cool completely.
• For the frosting: Fill a medium saucepan about one-third full of water and bring to a boil over medium-high heat. Whisk the egg whites, sugar, 1/4 cup water, the corn syrup, vanilla, cream of tartar and salt in a large (preferably glass or metal; see Cook's Note) heatproof bowl that will sit over the pot without having the bottom of the bowl touch the water.
• Set the bowl over the pot and beat with an electric mixer on low speed until the sugar dissolves and the mixture starts to thicken, 1 to 2 minutes. If the mixtures starts to crystallize on the side of the bowl, brush down the bowl using a wet pastry brush.
• Increase the mixer speed to medium-high and continue to beat until the mixture is glossy and holds firm peaks, about 7 minutes. Remove from the heat and continue to beat until the mixture cools slightly, about 30 seconds more. Beat in the lemon juice and 2 drops of pink gel food coloring. The frosting will start to firm up even more as it cools.
• While the frosting is still slightly warm, start to assemble the cake. Place the most vibrant pink cake layer (tinted with 1 teaspoon food coloring) on a cake stand or serving platter. Spread a heaping 1/2 cup frosting on top of the cake into an even layer using a small offset spatula. Stack the third most vibrant pink cake layer (1/4 teaspoon food coloring) on next, followed by another heaping 1/2 cup of frosting. Repeat with the second most vibrant pink cake layer (2 drops food coloring) and another heaping 1/2 cup frosting. Then top with the lightest pink cake layer (1 drop food coloring). Spread the remaining frosting on the sides and top of the cake, using the offset spatula to create a swirled design. Do not refrigerate; the frosting will set once completely cooled, about 30 minutes.