Preheat your oven to 350°F. Grease and flour two 9-inch cake pans, lining their bottoms with parchment paper cut to fit. Set aside.

In a small bowl, beat egg whites, 1/4 cup of the buttermilk, and lemon zest until combined. Set aside.

In a mixing bowl, beat softened butter and sugar until creamy, about 2-3 minutes. In a separate bowl, whisk together dry ingredients. Gradually add dry mixture to wet mixture, then slowly incorporate egg white mixture and remaining buttermilk. Mix until smooth.

Pour batter evenly into the prepared cake pans. Bake for 25-28 minutes or until a toothpick inserted in the center comes out clean. Allow cakes to cool in pans for 5 minutes, then invert onto wire racks to cool completely.

For the frosting, beat butter for 3-5 minutes until pale. Add powdered sugar, lemon zest, and lemon juice, beating until fluffy. Remove 1 cup for filling and set aside.

Mix heavy cream into remaining frosting and beat for an additional 2-3 minutes until light and fluffy.

Combine 1 cup of frosting with 1/4 cup lemon curd to create the filling.

Assemble the cake by placing the first layer on a serving plate. Spread lemon curd filling evenly over it, then top with the second cake layer. Frost the top and sides with remaining lemon frosting.