Preheat the oven to 350°F. Spray 2 9-inch cake pans liberally with baking spray (like Baker’s Joy that has flour in it.) Place a parchment paper liner in the bottom and brush more baking spray on top, set aside.
In a medium-sized bowl, whisk together the flour, baking powder, and set aside.
Add the butter and sugar to a stand mixer with the paddle attachment and mix on medium speed until light and fluffy (about 2 minutes.)
Add the egg whites into the bowl in three batches, mix thoroughly in between each addition.
Next, add the flour mixture and stir it in until just combined, scraping down the sides.
Lastly, add the buttermilk, lemon juice, vanilla extract, and lemon zest.
Stir it in until combined. Scrape down the sides and give it another 10 seconds of mixing.
Divide the batter amongst the two cake pans and smooth out the tops. Bake for 22-27 minutes until lightly golden brown on top and a toothpick inserted into the center comes out clean, a few crumbs on the toothpick is okay, any wet batter and it needs to bake longer.
Let the cakes cool on a wire rack in their pans for 5 minutes then turn out the cakes and place them (top side up) on the wire rack to cool completely.
While the cakes cool, make the frosting by placing the butter in a stand mixer with the paddle attachment. Cream the butter until smooth (about 30-45 seconds.)
Add the powdered sugar a little at a time until fully combined, scrape down the sides as needed.
Stir in the lemon juice and vanilla extract until incorporated. Place the speed on medium-high and whip the frosting for 3 minutes until light and fluffy set aside.
To assemble the cake place a tablespoon-sized dollop of the buttercream onto the bottom of your serving dish or cake plate, this will help keep the cake in place while you assemble. Take the parchment paper off of one of the cakes and place it (bottom side down) onto the buttercream on your serving platter.
Place the lemon curd on top and smooth it out to the edge leaving a half-inch border.
Take the next cake and take off the parchment paper liner and place it top-side down on top of the lemon curd. Lightly press the cake down to distribute the curd out toward the sides but not so it is pouring out.
Frost the cake with a small amount of buttercream for a crumb coat. Place in the refrigerator for 2 hours to set (place the extra buttercream in the fridge too to use after the cake has set.)
When ready, frost with the remaining buttercream, slice and serve.
Notes
Make your own lemon curd if you'd like.
If you want to freeze the cake, freeze the cake layers only.
Decorate the cake how you like. I used a Wilton 1M piping tip to create the swirls at the top.
The crumb coat is important to keep the crumbs from getting into your frosting.