To prepare cake, grease and flour two 9-inch cake pans and line bottoms with parchment paper (cut circles to fit). Set aside.

In a small bowl, beat egg whites, 0.25 cup of the buttermilk, and lemon zest. Set aside.

In a mixing bowl, beat 0.75 cup unsalted butter and 1.75 cups granulated sugar until creamy, about 2-3 minutes. Add in cake flour, baking powder, and kosher salt and mix until combined. Slowly add in the egg white mixture. Beat in the remaining 0.5 cup buttermilk.

Pour the batter evenly into the prepared cake pans. Bake in a 350°F oven for about 25-28 minutes, or until a toothpick inserted into the center comes out clean. Remove and cool in cake pans for 5 minutes. Invert onto a wire rack and cool completely before frosting.

For the frosting, beat 1 cup unsalted butter for 3-5 minutes until pale in color. Add 4 cups powdered sugar, lemon zest and lemon juice (about 2 Tbsp from fresh lemon). Do not add the heavy cream yet. Beat for 3-5 minutes until fluffy. Remove 1 cup of this frosting mixture and set aside for the filling.

To the large bowl of frosting, add 2 Tablespoons of heavy cream and beat an additional 2-3 minutes until light and fluffy. This is your main frosting.

To make the filling, combine the 1 cup of reserved frosting with 0.25 cup of lemon curd and beat until combined.

Place the first cooled cake layer on a serving plate or cake platter. Gently tuck strips of parchment paper under the cake to keep the frosting off your serving dish.

Spread the lemon curd filling evenly over the top of the first cake layer.

Place the second cake layer on top of the filling and press down slightly to secure.

Spread the remaining lemon buttercream frosting over the sides and the top of the cake. Carefully remove the parchment paper slips. Enjoy!