Preheat the oven to 350°F. Spray 2 9-inch cake pans with baking spray, take a pastry brush and brush the spray all over to make sure it is even. 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 salt, set aside.

Add the butter and sugar to the body of a stand mixer with the paddle attachment and mix on medium speed until light and fluffy, 2 minutes. Add the egg whites into the bowl in three batches, mix thoroughly in between each addition. Add the flour mixture and stir it in until just combined, scrape down the sides. Add the buttermilk, lemon juice, vanilla, and lemon zest. Stir it in until combined. Scrap down the sides and give it another 5 seconds of stirring.

Divide the batter among 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, 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 the body of a stand mixer with the paddle attachment. Cream the butter until smooth, 30 seconds. Add the powdered sugar a little at a time until fully combined, scrape down the sides as needed. Stir in the salt, lemon juice, and vanilla 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 fridge for 2 hours to set.

Frost with the remaining buttercream, slice, and serve.