Preheat your oven to 325 degrees. Spray a 9×13 metal pan on the bottom, line with parchment paper, and spray again. If you need to use metal binder clips to hold down the sides, remember to use metal, not plastic. Set aside.
In a medium sized mixing bowl, whisk together the cake flour, baking powder, lemon zest, and salt. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, cream the butter, oil, and the sugar for 3 minutes on medium-high speed. The color will start getting lighter.
With a rubber spatula, scrape down the sides of the bowl and begin to add the egg whites and whole eggs on medium speed, scraping down the sides and bottom of the bowl between additions. Once all the eggs have been added, beat the mixture on medium-high speed for an additional 2 to 3 minutes. Add the yellow food gel if using!
Add the sour cream and the lemon flavoring of your choice and mix on low. If it starts to look a little curdled, don’t stress, once you add the flour, it will smooth out.
Turn the mixer to the lowest speed, add ⅓ of the flour mixture, followed by half of the buttermilk, repeat, ending with the flour. Remember to be on the lowest speed to prevent over mixing. Scrape the bottom of the bowl, and mix on low for 20 more seconds.
Transfer the batter to the prepped pan, and with the back of a spoon, evenly distribute the cake batter across the pan, and bake for 38-42 minutes or until a toothpick inserted into the center of the cake comes out with a few moist crumbs on it. Let cool in the pan. Let it completely cool before you frost it.
For the cake
I suggest doing this section of the cake a day before to ensure that it sets properly.
In a medium saucepan, whisk the sugar, lemon zest, and lemon juice. in a small bowl, whisk in the whole eggs and yolks until combined, and then whisk them into the lemon mixture. 1 cup granulated sugar, 1 Tablespoon lemon zest, ½ cup fresh lemon juice, 3 large eggs, 4 egg yolks
Whisking constantly, start cooking the mixture over medium-low heat until it’s thick like pudding, about 6 to 8 minutes. It will start to bubble, keep whisking and cook for about 30 more seconds Remove from the heat and whisk in the butter until fully incorporated.
Strain the mixture through a fine-mesh sieve into a small bowl, pushing it through with a rubber spatula. Press plastic wrap directly onto the surface to prevent a skin from forming. Let it come to room temperature and then refrigerate.
For the cream cheese buttercream
In the bowl of an electric mixer, combine the butter and cream cheese. Beat on medium speed for about 2 minutes. Scrape down the sides and bottom of the bowl. Beat again for 2 more minutes.
With the mixer on low, add the powdered sugar, followed by the cream, vanilla bean paste, and a pinch of salt. Beat on low just until combined. With a rubber spatula, scrape down the sides of the bowl.
Turn the mixer up to medium-high speed and beat for about 3 to 5 minutes.
With a rubber spatula or a wooden spoon, beat out all of the air pockets by hand for a smooth look.
Assembly
You can leave the cake in the pan. Once it’s completely cool, you can frost the cake with the cream cheese frosting. Add dollops of lemon curd to the frosted cake (you will only need about half of the lemon curd. Serve the other half on the side of each piece of cake). Swirl the lemon curd into the cream cheese buttercream. Serve at room temperature.