Preheat oven to 350F and prepare 2 8-inch pans by greasing the bottoms and sides and putting parchment paper at the bottom for easy release.
In a medium bowl, sift 2 ⅓ cups cake flour, 2 ¾ teaspoons baking powder and ¼ teaspoon salt. Set aside.
In a bowl using a handheld electric mixer or a stand mixer fitted with the whisk attachment, beat 5 egg whites on medium-high speed until foamy. Add ¼ teaspoon cream of tartar and whisk until you reach stiff peaks. Set aside
Using your fingers, rub the 1 ¾ cups sugar and 2 tablespoons lemon zest together until they are combined and the sugar slightly moist (and your kitchen filled with that wonderful lemon sugar smell). Set aside.
In another bowl, using a handheld electric mixer or a stand mixer fitted with the paddle attachment, beat ¾ cup butter and your lemon sugar on medium-high speed until light, fluffy and smooth.
Switch to low speed and add a third of your flour mixture. Beat to combine.
Add half of your milk and continue beating.
Add another third of your flour, the last of your milk then finally the last of your flour. Make sure each addition is well combined before adding in the next but do not over beat (your mixture will be thick). Also remember to scrape the bottom and sides of your bowl.
Gently fold your egg whites into your batter with a rubber spatula about a quarter a time. Be very careful not to deflate the air you've incorporated into the egg whites. You will notice that as you fold, the batter will become smoother and easier to handle.
Divide your batter equally and transfer to your two pans.
Bake for 30-40 minutes or until a toothpick inserted in the middle comes out clean.
Take out of the oven and cool in the pans for 10 minutes. Run a thin, sharp knife along the sides of the pan, take the cakes out, carefully remove the parchment paper and let cool on a cooling rack completely before frosting.
Add the 3 tablespoons lemon juice and beat until fully incorporated or until you get the consistency you want.
Trim the tops of your cake if they are uneven using a serrated knife or a cake leveller. The important thing is the tops are level and your two cakes are of equal height (you can eyeball this but a ruler is very handy).
Take one and place on a platter or cake turntable smooth side down (trimmed side up). Using a piping bag fitted with your favourite large tip (I would recommend the Wilton 1A or similar or just simply cut the end of the bag to make a hole about 1/3 of an inch in diameter), pipe a border around the cake to act as a dam so your lemon curd won't ooze out later.
Once you've done that, fill the centre with about ¼ cup lemon curd. You might need to add more - the key is making the height of the lemon curd equal to the height of your dam so your cake doesn't sag in the middle.
Put your other cake on top, smooth side up (trimmed side down). Now you can go crazy! Decorate as desired.
Making homemade lemon curd would make this a 2-day cake because the curd has to be refrigerated overnight.
You can also make lemon sugar ahead. The recipe for lemon sugar is super easy; I have a stash of it all the time.