Preheat the oven to 325 degree Fahrenheit. Prep 2, 8 inch round cake pans by spraying them with cooking spray, using a parchment round on the bottom of the pan and spray that again with cooking spray. Dust the whole inside of the pan with flour and tap out the excess.
In the bowl of a stand mixer sift together flour, baking powder, salt, baking soda, and 1 ¼ cups of sugar.
In a small bowl, mix the melted butter, buttermilk, vegetable oil, vanilla, lemon juice, lemon zest and egg yolks until combined.
In a separate, clean bowl of your stand mixer use the whisk attachment to beat the egg whites until foam, about a minute. Lower the speed to medium low and add in the remaining ¼ cup of sugar while the mixer is still running. Allow the egg whites to beat until stiff peaks form, just about a minute. Set aside.
With the bowl containing the flour mixer, use the whisk attachment on low and slowly add the butter mixture to your flour while mixing, about 15-20 seconds. Scrape down the sides of the bowl and the whisk and mix again at medium low until the mixture is combined, about 15 more seconds. Fold in about ½ of the whipped egg whites until combined and then gently fold in the rest, taking care to make sure it’s fully incorporated, and no streaks remain.
Pour your batter into the prepared cake pans. Bake for 30-45 minutes, or until the tops of the cakes bounce back when touched and a toothpick comes out clean.
Cool the cakes in the pans for about 10 minutes before flipping them upside down on a cooling rack and remove the pans. Allow the cakes to cool upside down on the cooling rack. This will make the tops of your cakes nice and level.
Once the cakes are cooled, wrap them individually in plastic wrap and store in the fridge overnight, or at least 4 hours. Chilling the cake layers makes it easier to frost without getting crumbs in your frosting.
When the cakes are done chilling, use an offset spatula to ice your cake with the frosting, using it for the filling as well.
Set a small pan on the stove. Put about an inch of water in the pan and bring it to a simmer over medium-low heat. In a mixing bowl whisk together the egg whites and sugar. Place the bowl over the water, making sure the water doesn’t touch the bowl.
Whisk constantly until a thermometer reaches 165. Remove the bowl and use the whisk attachment of your mixer to whip the egg whites on high until you have medium stiff peaks and the bowl has cooled to room temperature.
Switch to the paddle attachment and, with the speed at medium, add in the butter 1 tablespoon at a time until it is all incorporated. Add in the lemon zest and salt and turn the speed up to medium-high. Mix until the frosting becomes thick. It may take a bit and it may look like it has curdled but trust the process and keep mixing! It will come together for a nice smooth, rich frosting. Keep the frosting in an airtight container at room temp until the cake layers are ready to be frosted.